Dual spec(well, at least before the insane gold inflation of MoP beating the static 50g fee).
I couldn't imagine playing cata/wotlk without dual spec, and BC would've been a billion times more fun if I could PvP and Raid without paying a whopping 100g a week going back and forth, 100g was a decent chunk back then.
Autounmounting when interacting with 3d environment ... no clue why they brought back the need to unmount to start working on your farm.
The cataclysm broke the world ... and the pandas could not fix it!
Achivements most likely. This feature gave me alot to do. Loads of content, trough some off it is abit cheap.
Edit: After that its likely Battlegrounds and flying mounts.
One reason for WoW decline I often hear is that its an old game and naturally people get bored with it. But TV is much older and people still watch that. Surely if a thing you once enjoyed keept staying fun, you would still use it?
LFG/LFR > Dailies > Flying Mounts > Dual Spec > Transmog > Pet battles > Transmog > Achievements > CRZ > Account achievements/Pets
> Arena (I don't PvP :P)
I'd say LFD and LFR, finding dungeon-groups was a pain before them, and especially as plate DPS (Tanks would always stall and go "Soz full" when they had 3 non-plate DPS because they wanted to grab plate DPS gear themselves), not to mention the time it would take to find healers...
LfR because it made raids doable for me, i don't like being stuck to other people's schedules for my gaming so i never got into a raiding guild, but with LfR i can at least sort-of raid, before LfR it sucked to have raids tacked onto the game knowing i wouldn't see them (outside YouTube anyway) until at least the next expansion.
Account wide everything (well, a lot of things anyway). It made playing alts much more rewarding.
Also LFD/LFR, before that I wasted most of my time looking for groups. And it allowed me to play when I could/wanted, not needing to wait on others to come home from work, school or finish their dinner and whatnot. As most people have different schedules of their real life activities.
Sure you encountered more bad players, but I would take that any day over the damn waiting.
Voice chat could easily have been the best addition ever, if they didn't screw it up..
All the account wide things.
Transmog so I can look pretty.
LFR. It created a whole new endgame for casuals who didn't want to do normal raids. It's not just a cool new feature, it's the main reason I'm still playing MoP so much months into the expansion.
A few months into Cata, I stopped playing because there was nothing to do for the casual non-raider past running dungeons. Now, I'm busy running all my level 90 alts through LFR and leveling up a few more alts so I can run them through LFR. LFR is THE endgame for casuals now and so much more enjoyable than dungeons.
For me it was Looking for Raid at top slot.
Due my real life priorities these days I had to give up scheduled raiding late wrath, and figured that was it, I wouldn't see the inside of a raid again, so kind of resigned myself to that.
However now with LFR I can get to see these raids, while it is nothing compared to that feeling of raiding with my guild and friends on progression fights, it still allows me to have a small bit of character progression and see the content I never thought I would see again.
My second would be the pet/mount UI, because having to use up bag slot for each pet was a nightmare, and because I could never bring myself to deleting my wee cuties, i ended up deleting pvp gear.. crafting mats.. flasks/pots/food etc just so i could carry my fav pets and mounts around with me haha.
Oh and transmog, seriously even before it I would save any tier or just unusual bits of armor and make up sets for my toons for when they were in a city or when they were RP'ing or going to events, so for a long time I have filled bags and bank with all these lovely outfits, and now I get to use them all the time, it's fab!