In the too long rant below I provide somewhat stream-of-consciousness discussions of the specs/classes I know best along with thoughts on the game generally, in the context of rank-ordering my favorite specs/classes. Also, a controversial argument about needing, and being very able, to introduce a fourth type of pve spec that breaks out from the tank/healer/dps structure with ideas on how to do it without hurting those who don’t want their toons to make the change and which would be relatively easy:

(as an aside, I was... not sober when I wrote this novel, and being a PhD candidate I can write on most anything for far too long so I apologize)

[1] Elemental Shammy. I was first introduced to WoW--and MMOs as a concept--about a week after Cataclysm was released, so I'm relatively new to the game but have been around for 2 expansions now. In the past few years classes have undergone many changes. I leveled every class, in at least one spec to at least level 35 within weeks of beginning to play. I then chose two-an elemental shaman that briefly flirted with enhancement and a frost mage that learned fire later in the game-as those I most enjoyed and started created new toons to play them as. I loved frost; it was my first 85 but as I hit the 50s on my shaman I began to love it more and more. Always underestimated, ele/shammys are versatile, unique range casters that are about the closest thing of any spec to a true support-oriented spec. The class has changed over time and, while some decisions I disagree with, I've found most of these changes to be for the better. I think we're underappreciated, but a good elemental shammy is a very powerful toon. I never get tired of playing it since and while my mage still sits at level 87 with 3 90s ahead of it it, my elemental shaman is my best geared toom

[2] Affliction Warlock. I've had a long love-hate relationship with both affliction and destruction warlocks. Affliction is an incredibly appealing spec on a conceptual level, but I found it tedious throughout much of Cataclysm. It took too much focus on very technical mechanics away from just playing the game in a fluid way. But MoP revolutioned my second 90 and Affliction is much more enjoyable than before, with real room for decision and customization. They've fiddled with the spec along the way, and I do often play my lock as destro, but I've enjoyed all of the specs iterations throughout MoP. There are still some major kinks to work out: as the premier dot class, I find Affliction's lack of dynamic stat updating for dots a major issue that should be resolved asap and the classes association, and use of, spells of unique defensive nature (fears, psyfiends, psychic horror, mortal coil to name a few) it still feels like they could be made more important in high-level pve and help flesh out the support/dps role I mentioned earlier far better. This set of defensive spells, as well as pets, still are missing... something; they aren't fully coherent and they lack direction. I think that WoW needs to start thinking about a 4th spec that comes along on raids and dungeons, ones where dps is important but not of utmost importance and are instead valued, in part, as a result of their ability to provide unique support to themselves and others. Shaman do it with totems and passive abilites currently, though could use a more sophisticated approach to totems. Aff locks feels like it could be such a spec if only allowed to capitalize on these abilities; indeed, the actual nature of dots strongly lends itself to this type of gameplay approach: provide several spells that eat away at the targets health over an extended period of time while you engage in other activities. But now Aff is a bit overwhelmed by Haunt, the channeling of soul drain to recharge soul shards which still sometimes feel like an overly cumbersome mechanic on their own, the channeling of soul drain when the target is below 20%, the the channeling of malefic grasp all other times... it leaves no time to make use of these abilities, which often dont work in raids or dungeons anyway as such support abilities have never been well integrated into raid fight (and role-perception) mechanics. However, the class remains a blast, highly unique and headed in the right direction.

3. Destruction Warlock. Favored over affliction towards the end of Cataclysm, and off and on during MoP (likely most during 5.2), my love of this spec has led to my warlock being the only toon I regularly play using two different specs. But my biggest complaints are, interestingly, that the class was overly complex (and a bit clunky) in Cataclysm and currently is a bit on the easy, underdeveloped side. I enjoyed the complex play of destro in Cataclysm, it made it a very hard spec to master and made it very satisfying to do well as. But at the same time, the class seemed too tied to Affliction mechanics, it wasn't too tied to the central features of warlocks generally. Most damning was the payoff for mastering the spec: put far more effort into playing well and still not be easy or powerful as a mage. In MoP the class was simplified, too much so in my opinion--though more so earlier in the expansion. The spec's repetoire needs another good run through by developers; don't change anything substantially, but offer a bit more variety in rotation options and utility. I'm glad that Destro was allowed to come into its own this expansion, separated from affliction to a much greater degree (no longer is is simply a fire mage's enstranged cousin) but the results need a bit more work: provide some new spells and alternative rotation choices. But, overall, still close to affliction and a bit behind elemental shammies in terms of favorite specs.

4. Boomkin. Hated'em. Hated them for a long time. The one dps spell caster I just couldn't enjoy no matter how much I wanted to. The eclipse system was just awful in Cataclysm. I think it still hampers the spec a bit, but it has been made more dynamic and fun; I don't want Blizzard to do away with it, just continue working at streamlining it. However, overall I'm finding my balance druid (level 85 as of a couple days ago) very enjoyable this expansion. This is the toon I've leveled most seriously in almost it's entirety this expansion - most of the toons I still play began life and were well leveled by MoP but I had deleted my boomkin because I hated it that much. It feels so much more fluid now; it is the best utility spec (which is clearly something I'm interested in) in the game. It is the most powerful healer of any of the so-called 'hybrid classes', rivaled only by Spriests as a dpser that can do viable backup healing to anyone aside from themselves (ret/pal, ele/shammy, enh/shammy all have some nice healing abilities compared to other dps specs, but their utility to providing healing to others is lacking - great for survival when the healers' attention is elsewhere - but that's about it). The eclipse system is now manipulable and feels more interesting and less tedious than before. AoE is interesting. Most promising is the hope that the current class as a whole's direction has been steadily followed this expansion: the shapeshift forms of Druids are taking on utility off-spec, sometimes being a cat is useful even as a Boomkin. Some talents and glyphs are allowing players to try and become real shapeshifters that may have reason--and the ability to usefully--shift into an 'off-spec' form and do so easily. This line needs to be followed, each of the four specs should maintain its current focus, but they should have unique ways of using each form.

5. Retribution Paladin. My first toon ever was a human retribution paladin on a pve server. I enjoyed my pally for a long time, as I learned game mechanics and leveled (very slowly, given both inexperience and the lack of heirlooms or gold from alts). However, the paladin never quite lived up to the 'spell-casting knight that could fight effectively in melee or at a distance' reputation I read about when trying to choose my first class. I've never been great at melee and I found the paladins use of 'hands' and other types of spells rather unenjoyable (though the utility they provide is interesting and therefore make this another spec that could--and is--progressing in a way that favors not only dps, but general utility). I left my Pally shortly after arriving on Northrend when I decided that a mage and a shammy were going to be my first 85s. I recently leveled him to 80 and found it enjoyable, I like a lot of the changes and the talent choices available allow for more customization than is possible with some classes. I'm still not the best at melee, but now we have the option of glyphing so as to make Word of Glory extremely powerful as an offensive use of holy power and exorcism remains a powerful distance attack - retribution paladin are becoming more versatile and, like enh/shammys, have a lot of potential to be true 'magic fighters' that mix distance spells with melee magic attack. I'm looking forward to leveling him the last 10 levels once I finish my druid.

6. Spriest. I began a spriest late in Cataclysm and leveled it from roughly lvl 40 to 90 after MoP arrived. The spriest was fun, and easy, to level and has always seemed a natural choice for pvp (something I barely do, though I'm now started to invest in making my spriest into a primarily pvp-oriented toon). DPS, however, is lacking. And recent nerfs don't help the situation; I'm not sure where Blizzard is going with Spriests. They are, and should be, the most powerful off-healers of any dps spec. They can fight for a long time without dying or being focused on by dedicated healers. They can provide defensive spells and strong heals when necessary. But they are a dps class and insofar as that is the case, they seem to be getting weaker and less relevant and, honestly, I don't find them incredibly interesting. I still play mine often as it is my 3rd level 90 and I still enjoy it, but the rotation lacks variety moreso than most and, in some ways, it also feels like a dumbed down, less effective caricature of an affliction lock. Spriests need more attention and greater choice in customization via talents and glyphs. They should be able to focus on a few heavy dots, the occasional mindbender and procced instant cast spells (mind blast, shadow word: death) for higher dps while filling in gained time to be truer priests - allow us to shape them to be good dpsers that can provide real off-healing and provide strong support to others or excellent dpsers that have high survival power. These are dark priests! Make'em into a spec that has strong dot and instant attacks while allowing them to have access to more of the priest repetoire for healing, use glyphs and talents to soften how powerful this could be and allow customization in how its implemented. I still like my spriest, I just hope that the spec doesn't continue to be ignored as the one spec from a predominantly healing class that does dps so it's easier for priests to level... it just doesn't get enough attention or development despite being very promising (could say that of marksmanship & unholy dks--hell, dks generally--too).

Those are the toons I know well enough, and enjoy enough, to play regularly and plan to maintain in some way. A few special cases:
A. Frost Mages. Mages were perhaps overpowered in Cataclysm (and I hear they still are in pvp) but they were fun, and they were an easy class, not just spec, to play (I've actually never played arcane, so I haven't tried what is supposedly the easiest spec to play while also being one of the highest dps specs in the game). My mage was first to 85, I did it all as frost and still consider that the best leveling experience. It was fun, it was survivalist without any substantial heals, and it had enough complexity to stay interesting. Unfortunately, when I hit 85 and began to learn more about serious pve frost quickly became untenable – I simply used the spec for skinning and herbing. I played fire some, but found it somewhat unenjoyable (and still do, though every once in a while I might play it for… 10 minutes or so…) MoP supposedly made frost more pve viable and many seem to enjoy the changes but I basically hate everything about playing as a mage in MoP. They took one of my two favorite classes and ruined it for me personally (though I recognize some find it appealing). Frost is extremely clunky, when I play it I feel like I must not being doing it right, that I must be missing spells, that I don’t get the rotation. However, I know the spells and rotation… and I know I’m doing most things correctly (for what little I’ve played). I just find it halting, uncontrolled, poorly conceived and lacking the ‘fun’ that was frost in Cata. Please, streamline the spec, do something about the god-awful water elemental and figure out a way to make frost’s uniqueness more relevant and worthwhile in high level pve.
B. Hunters. I’ve a level 48 hunter that I’m finally starting to enjoy; I clearly like spell casters but for the longest time found hunters to be caster wannabes. However, as I’ve given it more time I’ve found it more enjoyable and looking ahead towards later abilities I think it will be great to get leveled. But I can see some problems: Beast Mastery is the ‘right’ choice; it’s dps is typically top, it’s easier, it’s quick and efficient. Yet, WoW has never quite figured out pets. Warlock pets are just sorta there, and only a few other specs have anything resembling a pet. But Hunters are about the pets and Beast Mastery is focused directly on them. Pets need to be much more customizable; and choices about them should actually be less easy to change (the ability to easily change glyphs, talents, specs, etc. has generally been interesting and fun, but it has gone a bit too far at times). I believe catching, taming, naming and dedicating some actual time to customizing and building up pets, at least as Beast Mastery if not Survival or Marksmanship, would make hunters far more interesting and enjoyable. Pets need to have more abilities, they need to be smarter and they need to be easier to control. As for the other specs, I have only played Survival a little but it seems fun, if not to exciting. Blizzard has admitted to some blandness and a desire to give hunters the treatment warlocks received this expansion and that would certainly help. Marksmanship is just in a sad state, reviled by pve (and many pvp) players everywhere, they have no flavor, no uniqueness. Back in Cata, Marskmanship drew me to hunters but they are currently a blank slate. A class in need of help (with one strong spec that suffers from a continued inability by Blizzard to find a way of better customizing and integrating pets.
C. DKs – I want to enjoy them, never made it past level 61 and have tried many times. Unholy appeals to me, but I just don’t seem to appreciate the playstyle. Of course, melee isn’t my thing and warlocks overshadow them for me personally… Of course, if I gave them more time I might enjoy them more…
Long rant. Tangential conclusions: give us a fourth spec role/type for pve! Real hybrid, support, dpsers. Many specs in many classes could easily adapt, and if the glyph system and talent system evolve in particular ways they could still serve as pure dpsers to those who have enjoyed them as such for a long time. I believe elemental shaman, boomkins (make the magic druid spec a spec more tied to shapeshifting generally, if shapeshifting isn’t magic nothing is), affliction warlocks, retribution paladins could all make the transition easy. Hell, don’t just expand upon the glyph and talent system to allow for this, do it for all specs in ways that allow for increased customization and which weaken the traditional ‘tank, dps, healer’ model. Beyond that, fix mages please, find that last little spell to round out destro (and streamline embers a bit more), and go ahead and make ret/pallys and enh/shaman true magical fighters! Additionally, revitalize the professions and let us buy (and make) enchants and gems that are more innovative in effect – stats aren’t everything, the enchantment and gemming systems both could be harnessed in ways to offer extra talent/glyph-style customization to make it easier to develop these hybrid support-style toons.