Really close for me between achievements and transmog. I change my transmog on a near daily basis. However, without the achievements, I would have no reason to play this game outside of raid and would quickly grow bored with it I'm afraid, so that's where I gave my vote.
Transmog! (i think it's more fun to design transmog sets than actually trying to get new gear)
Personally i don't care about achievements at all and would be perfectly okay if they were removed completely. Can't see the point (other than e-peen thing or proving you killed heroic bosses). "at last i finished that achievement where i needed to jump down to lava naked!!! i've completed this game now though i can't even remember which was the name of the last boss!"
I don't like: raids, grouping, warcraft movie, doomhammer
I hate: orcs, especially thrall, pvp, sylvanas, legacy server whiners
I like: ???
I don't know if the game would have held my interest for as long as it did without them.
Of course, prior to WotLK, the game had no need of them as there was still plenty of immersion and things to do that actually took time.
Achievements are the best! They encourage you to do some of the optional content available in the game when you become bored. IF there's an achievement or FoS for anything, I've got to get the majority of them! PLus I have another reason to complete old quests besides lore!
Isn't it ironic how education is important, yet people forget all about it when they visit the internet?
As much as I love transmog and Achievements, for me it was when LFG/LFR came out. Before then I was always missing out on taking part in the whole experience, but all of a sudden I didn't have to wait to be invited to someone else's run, I was able to jump right in. I became a better player and more confident and WOW was a whole new game for me.
Achievements, I'd be terribly bored without them. Transmogr is a good second for me, don't really care about the other features
wow voice chat.
The gear in MoP is pathetic to say the least. Look ridiculous. Yes yes, art is subjective bla bla bla.
"The sword is mightier than the pen, and considerably easier to kill with."
Sorry but LFG/LFR. I'm pretty social too.. But I have friends irl/job/family, and I don't need some ingame "friends" to do stuff..
dual spec best feature
along with flying ( you see what I did there)
Achievements system: I was okay with that, but now with all the LFR collapsing the community It's hard to get a group for raiding achivement,
Reforge: I gave me headache and a pain in the ass everytimes you get 1 new loot
Transmog: Cool feature
LFG/LFR: LFG is okay for 5 men's, LFR by far the worst thing in this game
Flex: Don't care, it will take more of that to get me into raiding