I put my vote into Class Mechanics.
I love my Priest to bits. She's my very first class. I used to be MS Shadow before I joined my main raid group to heal. I also have an 85 Pally (Holy/Ret/Prot), Druid (Boom/Resto), and Shaman (Resto/Ele). My Priest is the only class where I comfortably and really enjoy playing all 3 specs (I have to raid in all 3 specs too >< yay for constant respeccing!)
For healing, I love the spell variety. Some say there's a lot of spells for healing priests but I don't think so, or maybe I'm just used to it because it's my first healer.
For Shadow, what's not to love? Except maybe RNG orbs... ><
- PW:Shield - Amazing to have in all specs. Many lives have been saved with my precious bubbles! I hate being on like my Druid or Shaman seeing someone about to die and not being able to throw a little shield =[ I didn't mention Pally because they have LoH,which is a bit stronger than my PW:Shield =P
- Which leads to my love for Divine Aegis. I love absorbs. And my healing buddy always calls it my "cock blocking bubbles" XD Love building up max Aegis bubbles on fights like Zon'ozz and Domo before the fight and watch my group take no damage =P
- Buffs are a piece of cake, never need to worry about what buff to bring or who to give a certain buff to ^^ I guess that can also be considered a downside, since Priests don't give much in terms of buffs to a raid group.
- PoM is another one of my favorites, quite unique to our class.
- No CD on primary AoE heal. One reason I don't like healing on my Druid much is because AoE healing spells all have a CD (and Druids tend to be raid healers), while my Priest has the very wonderful PoH with no CD. I had no complaints back when Holy Pallies had a CD on HR because Hpallies were never really considered as raid healers and were given a fairly strict role as tank healers.
- Atonement and Smite Healing - I leveled my main as Shadow/Holy and didn't touch Disc until I was 85. When leveling my second Priest, I decided to MS Disc and check out Disc leveling since that is something I missed out on my main. It was wonderful and strong, plus no mana problems like I had with low level Shadow. Atonement was amazing at low levels. At 85, I do swear by using AA/A spec. I don't like downtime / don't like to stand around doing nothing, and being able to DPS/heal/build Evangelism stacks is perfect to fill gaps of time for me. I don't like to spam PoH to build Aegis stacks during fights unless it's very predictable (like on Zon'ozz, not like Madness).
- No dependent CCs. I'm pretty lazy XD but this can be seen as a positive or negative, positive in my case. I rarely hear a request to shackle or MC something (and most of the time I can't do either!) It's always sheeps, hexes, hibernates, traps, etc. I can just chill out and twiddle my thumbs as the CCs go out 0=]
- Channeling. I love Mind Flay and Penance to bits. I love the ability to channel spells, where you could possibly get a tick off before having to move / be interrupted / etc. I've been leveling my little Warlock and I have the desire to just Drain Life all the things rather than deal with cast times of Shadow Bolt. That's one reason I'm not terribly fond of my Boomie, no fun channels =[ and Starfire likes to take years to go off. I also have a bad habit of spamming my cast spell rather than waiting for the right time to cast again, which I can't expect to do with a channel like Mind Flay so I end up focusing more on my button pressing.
- Shadowform and purpley goodness. I think when I first started playing (RAFed with a friend who invited me to play), I was debating between a Shadow Priest and a Hunter. I went with a Priest when I found out about purpley Shadowform. It was like a moth to a light, I must have Shadowform! Now I use Glyph of Shadow =X but even so I love the subtle shadowy look.
- Hymn of Hope and giving others mana - I mean, it's no Mana Tide and the CD isn't the best, but the temporary mana increase plus the ability to give mana to others while you're in any spec is great. Whether I'm healing or DPSing a fight, I naturally have an eye on the other healers' mana to time when I should mana hymn. Currently have no Druids in our raid for Innervates, and when we do it's either a feral tank or Resto Druid (who's currently healing on his Resto Shaman, and uses Innervate on himself on his Druid). Having two Priests and two Hymn of Hopes does wonders for healers in our raid. I hope it doesn't become spec exclusive.
- Holy Nova - Okay I only wanted to mention Holy Nova because it's pretty XD I like the graphics, it's a shame it's not more useful, although I do use it on H Madness. I choose Holy Nova > Mind Sear on adds because I want to be able to heal as well and not punish the other healers too much ^^ and I don't mean necessarily the heals from HN, but the ability to just HN like 2 times and then PoH to help heals, compared to having to stop Mind Sear, a semi expensive spell, after a few ticks.
- Binding Heal is another one that I love (though situational) and is also unique to our class.
- Mass Dispel - I would also add the ability to dispel in a DPS spec, but not with regular dispel anymore =[ I miss the ability to dispel friendly targets in Shadow. Makes me sad.
I'm just going on and on about Priests and like every spell almost =X. I have all healer classes and slowly working on a Warlock, Hunter, and Mage. While they're fun to play with and try out, at the end of the day, my Priest is where my heart is.
A Disc/Shadow/Holy Priest