So I've spent a fair few months in Legion Alpha/Beta and I decided to make a little overview over what I consider good and bad about the expansion, how it rates compared to previous content, and its potential staying power. At this point almost nothing will change in Beta anymore except for numbers and bugfixes, so it's as good an idea of the live content as we will get.
First off: I have played ever since Vanilla all through to current content, and I am predominantly PvE-oriented and a Mythic raider. This just to put my opinions in context. I won't comment much on PvP, for example.
Disclaimer: this is all personal opinion. Feel free to weigh in with your own pro/con tally.
The Great:
Artifacts
I have personally wanted an alternate advancement system/paragon leveling in WoW for a long time. I enjoy these types of meta character progression in many other MMOs and they give the game a lot of staying power and long term motivation. They are beautifully designed, most of them add a bit of depth and "oomph" to the specs and they are tied to some great lore and PvE content. While they come at the cost of not many new weapon skins in Legion, I feel they add a lot to the game. A definite plus in my book overall.
Questing
Following the development of questing from MoP and WoD, it's probably the best questing we've had so far. It incorporates all the elements that were good in the past, whether that be themepark-like exploration and treasure hunting, or the very well done on-rails story experience. There is more voice acting in general questing than there has been in the past.
To top it all off, we have endgame questing content in Suramar now. Suramar is truly an achievement and an experience to play through. It is up there with some of the better solo experiences in other MMOs I've had, such as SWTOR, and gives different types of players something to do besides grind/farm/instanced content.
Mythic+
You asked for it, now you got it. WoW finally has a viable PvE character progression for 5 man content. If you can't be bothered with raids at all, but still want challenging group content, Mythic+ definitely delivers. It draws strongly on the Greater Rift system from Diablo3, and gives dungeons a lot more staying power. It is appealing to more casual players and raiders alike, and simply adds to the palette of things to do in Legion.
World Quests
This system for max level activity is pretty much perfect, in my opinion. Quick and easy rewards that you can access fast, an alternate gearing path you can take if you dislike normal/heroic dungeons and/or crafting, as well as a daily activity system that is both more varied and rewarding than the old system of grinding 25 repetitive dailies.
A return to more PvE content
3 raid Tiers again, more dungeons with one extra one already confirmed for 7.1 and probably more on the way down the line, Mythic+, World Quests, Suramar - honestly I can't think of anything else I would want. Legion is giving me everything I missed in WoD, and started to miss in MoP a little bit, and then some. Big, big bonus points.
The Good
Raids
Anyone that has actively raided throughout the past 10 or so years will probably agree that in terms of encounter design, raids have gone only up and up each expansion. Legion is no exception, and adds to that chain of great raiding content. I have nothing bad to say about Legion raids, and cannot wait to tackle them on live.
Crafting
Crafting is actually fun now. Not only do you get some extra stories and lore to play through as you level, I also enjoy the new system of destroying average products to create higher ilvl. Add in that rare crafting mats are BoP and no longer tradeable, it shifts the weight of the market to active crafters again, which is always good.
Transmog
Already on live now, and also directly taken from Diablo, it gives old content a lot more replayability. Simply no negatives to this, it was long overdue and it is very welcome indeed.
Lore & Story
Personally speaking, I was kind of irritated with MoP, that sort of "took a break" from most of the ongoing plotlines, and was full-on exasperated with WoD, which just completely gave a giant "f*ck you" to them. I could not be happier with the veritable flood of actually interesting lore that matters that we are getting in Legion. Just leveling to max tons of exciting stuff happens, and Suramar is actually interesting because it directly ties into the history of several ongoing meta threats and looming story arcs. It just feels good to be "back on track".
PvP
I've played PvP more or less competitively, most notably in Vanilla and in the first few arena seasons, but I sort of grew bored and tired of it over the latter half of WoW's lifetime. Mostly thanks to the extra gear grind and horrible balancing, I guess. Legion doesn't really solve the balancing issues inherent to the game system that will always be there, but it does finally do away with PvP gear, which definitely makes me want to play it again occasionally, so it's already a success in my book.
The Bad
Class Halls
I'll be honest, I don't see much that's bad about Legion after playing it for a while. Class Halls is one of those not-so-great aspects.
It's important to note that Class Halls really are NOT like Garrisons from WoD, which were atrocious. There are no profession buildings or other buildings, you don't hang around in it on your own all the time, and it doesn't flood you in gold anymore. In fact, Class Halls actually make you go out into the world - a lot. You want resources for your Artifact Knowledge work orders (more on that later), for your missions, for Class Hall upgrades. And those missions in turn actually lead to new quests. So it's not horrible. But overall I don't see the need for the follower/mission system at all, especially in Legion with the metric tons of content it offers to do. It feels tacked on, unnecessary, but thankfully not as intrusive as it used to be.
Artifact Experience
While I really enjoy having a paragon leveling system, the way it was implemented poses significant problems. Specifically, the way it affects secondary specs and alts. Simply put, in order to be optimized for high level raiding, you need to focus completely on your main spec, and your secondary spec will inevitably suffer and lag behind. I would have far preferred if AP were allocated to all of one's artifacts equally whenever one receives any. The way it will go live only serves to dissuade raiders from playing their secondary specs much at first or investing into them, and I'm rather sad about that.
The Ugly
Specs (some of them, at least)
This depends on your personal preference and what spec you play, but while some specs (thankfully most of my favourite ones) have been revamped nicely and are great fun to play, several specs in Legion feel utterly gutted to me and are frankly appalling to play. I've utterly lost interest in my Hunter alt, I was never really into Rogue but Legion made it even worse, and Discipline has turned into a very, very niche spec that will appeal to only a small group of people. I could have put this further up and talked more about the specs that are actually quite fantastic - which is the majority. But I feel that the fact that there are quite a few devastatingly dull and uninteresting specs is more of a detriment than the ones that are enjoyable are an asset to Legion.
Summary and Final Verdict:
There are a few things I haven't mentioned that don't affect me, but are generally considered positives. For example, LFR has Tier loot again. I don't really play LFR except very rarely on my lowest-geared alts, and I don't really consider it attractive content for...well, anyone. But I guess the people that do enjoy it have something to look forward to again. Legendary items are fairly divisive for some, personally I'm totally neutral on them. They just add something else to hunt for and be occupied with, and they won't break balance any more than basic spec tuning/design would in PvE. And finally flying is going the way of WoD again - something I am more in favour of than against. Flying is nice to have, but I do believe it significantly detracts from the zone experience, and I'm fine with getting it later once I've seen and done everything and just want to get from A to B faster.
Overall I find Legion to be a return to form for the game. I've more or less enjoyed MoP, I endured WoD. I raided actively in both expansions and thoroughly enjoyed that, but the rest of the content was "eh" in MoP, and simply not there in WoD. Legion makes me want to play again, as in doing everything, experiencing everything, and finally offers a lot more than just raiding again, at least for me. PvPers may be disappointed that there is no new BG (again), and balance may be bad again, but at least it is more attractive to more players again without gear requirements.
I honestly consider Legion to have the potential to be one of the, if not the best expansion to date. The few annoyances that plague it could still be remedied down the line, and the good far outweighs the bad in my opinion.
Maybe you gleaned some information from this wall of text you were looking for. In any case, here's to a hopefully successful expansion launch!