Everyone who says BC will immediately be ripped to pieces by everyone else, saying 'yeah of COURSE you'd say that, nostalgia much?' etc. Please stop to think that it might actually be true, for them at least. I too voted BC. Everything seemed like a challenge, and that's why I play games. Arenas were new, and more like chess than dodgeball. Heroics were heroic. Raids had defined "progression" - Kara - Gruul - Mag - SSC/TK - HJ/BT-SW. Sure, it wasn't exactly easy to catch up to the curve (until they brought in BT-level badge gear after BT, at least). Raids were HARD, in general. Reps took a while. Realms had amazing communities that disappeared almost overnight when LFD came into play (at least the ones I played on - Thaurissan horde, Frostmourne alliance, Dath'Remar alliance, Proudmoore horde).
Vanilla was chaotically imbalanced, and didn't even pretend to be balanced. Every class was completely and utterly different - that's something I'll miss. I won't miss how pointless some specs were, or how utterly essential others were, how some were brought solely for buffs etc. But I enjoyed the hell out of it. Premade BGs in vanilla were my favourite times in WoW so far. Raids were great, although being stuck in Twin Emps for months in two different guilds kinda got to me. Dungeons were long but rewarding, and you really bonded with the people who you sat with for hours through BRD etc.
I loved and hated WotLK. It was a shock going from hard raids/dungeons to ones which took no thought or effort whatsoever. Sarth 3d being the only remotely challenging encounter of an entire tier was stupid. Ulduar was amazing, as nearly everyone agrees - I think it'll LONG be remembered as Blizzard's best raid, besides Naxx 40 for those lucky enough to see much of it in Classic. ToC was utterly pathetic as far as I'm concerned, though at least Anub hardmode was a challenge. ICC was beautiful, good mechanics, good balance of bosses. I wasn't a fan of the over-time nerfs, but there would have been an absurd amount of rage if everyone didn't get to see something that everyone's been looking forward to since release. Thought reps were pretty pointless in WotLK - dunno how putting a tabard on and spamming dungeons is supposed to make people on the other side of the world like you more. Dead men tell no tales, and all that sort of thing. I hated arena in WotLK. I'm not the kinda person who finds instagibbing people inside a shadowfury to be enjoyable or compelling gameplay. Too many cheese comps, one after another. Beast cleave, wizard cleave, lumberjack cleave, african turtle cleave etc.
Cata most people on here would be well aware of. I'm hoping to forget it existed as soon as possible. I liked BWD and BoT quite a lot, they were challenging and engaging. I hated all but Ragnaros of Firelands. Dragon Soul........ Dear lord that was a disaster. Clearing the entire instance on the first day in a PuG, on normal albeit, shouldn't happen. I'm glad they made a brick wall out of Spine heroic, although progression should never be made impossible without insane class stacking. That's just woeful design. Having the instance out for nearly a YEAR is almost inexcusable. PvP was at its lowest point in Cata, for mine. Dropped out of being an esport with things like MLG, which used to be real highlights of the year for me.
MoP's a mixed bag for me so far. Raiding's fine, but I'll never be a fan of LFR, especially while it remains for current tiers. For me, raiding's a journey - you WANT to kill a boss so you can see what the next has in store, so you can see its loot table, its mechanics, its RP, everything. I want to feel like I've earned the right to seeing it by killing its predecessor. LFR ruins that in one swift stroke. A bunch of randoms who you'll never see again, just plinking away aimlessly, and seeing all the content in one go? Ahhh it's sad. I know so many people love it, but not I. I like how reps are useful, but I don't like having to do them on every alt, since I switch mains about once every 2 months nowadays. PvP is UTTERLY HORRIBLE at present. Every spec has disables, stuns, fears, ccs, silences etc. On top of that, specs with amazing control can also instagib you at any point in time, even with amazing gear. It's so utterly and completely awful doing BGs in WoW's present state. 3s is ok, but still, being instagibbed by taste of blood, frost mages, bm hunters etc isn't exactly going to get more people into arena. Turn it more into decision making instead of popping your CDs and killing someone before you're killed, and more might enjoy it.