Vanilla: the freshness of the game
BC: ZA bear run
LK: dual spec
Cata: Transmog, maybe
MoP: too early to tell, but I'm having fun atm.
Burning Crusade - a new world, the exploration was really good
Wrath of the Lich King - death knights, although at start they were a class that attracted idiots because of being overpowered, now it's better and they're a good class
Cataclysm - transmog, always wanted to wear something nice but couldn't because of crappy look of some sets
Mists of Pandaria - challenge modes, with the dillution of heroic modes, this is a good thing brought.
MoP: Interaction with Lor'themar Theron in 5.1
TBC - Dungeons & Raids. They look dull today, yes, but at the time they were very nice mostly. A couple 5 mans in Serpentshrine Reservoir and one or two in the Netherstorm quickly got repetitive, but apart from those.
WotLK - Zones. This includes everything that goes with zones; scenery, music, design of the land, architecture, landmarks, general feel, etc. Nothing has ever been as epic as arriving in the Howling Fjord for the first time by ship. I literally thought to myself: 'Oh my, it feels like home... a thousand years ago.' I'm Swedish, by the way.
Cata - New starting experience. While I didn't particularly enjoy Cataclysm, there were a few things which stood out. Among those are the new starting experiences for our characters - a very welcome change which meant that you no longer facepalm and want to kill yourself for creating an alt.
MoP - A different theme. This might sound vague or dull, but I really appreciate that Blizzard went in another direction with MoP and tried something new. It'll also mean that next expansion (which will be far more important in my eyes) will do some classic Light vs. Legion shit, which is essentially what Warcraft is and should be.
"Good morning!" said Bilbo, and he meant it. The sun was shining, and the grass was very green. But Gandalf looked at him from under long bushy eyebrows that stuck out farther than the brim of his shady hat.
"What do you mean?" he said. "Do you wish me a good morning, or mean that it is a good morning whether I want it or not; or that you feel good this morning; or that it is a morning to be good on?"
"All of them at once," said Bilbo.
TBC - Raiding in general
WoTLK - Achievements
Cata - Vash'ir
MoP - too early to say
Bc - jc
Wotlk - grizzly hills music
Cata - those mobs that go 'woogaboogawoggilboogily'
Panda - so much to do so little time
BC: heroic dungeons and 10 man raids, first step in making all talent trees/specs playable/viable/enjoyable
WotLK: dual spec, LFD, all raids avaliable in 2 formats, music, storytelling and zone design
Cata: LFR, transmogrifying, old world rewamp
MoP: zone/world design, storytelling and immersion, music, challenge modes, account wide stuff, shitloads of stuff to do - probably more stuff to come, still too early to tell
TBC: the best state of PvP ever in WoW
WotLK: improving a lot of TBC flaws while still retaining challenging (and in Ulduar and ICC cases, absolutely stunning) content.
Cataclysm: I don't think anything is worth mentioning...
MoP: I'm not playing anymore, so I can't say. I'd probably like Pandaren.
MOP: no idea for now.
Vanilla : No flying mounts
Burning Crusade : No panda
Wotlk : No panda
Cataclysm : No panda
MoP : Not vanilla
TBC: Flying in Outland(s) and my server's awesome community (still awesome now, but it was the perfect server to learn on)
WotLK: First time I could do "real" raiding (apparently Kara doesn't count) and Ulduar
Cata: The revamp of the old world was just amazing [if you haven't leveled a character in the new world, you missed most of Cata]
MoP: Dungeon finder has been putting me in groups where 4-5 people are actually from my Realm [this I love and think should have happened from the start]
TBC - skipped it, now when i level through it on alts I hate it and just want to be done with it
Wotlk - story,ICC
Cata - 1-60 revamp
MoP - like someone before said, a different theme, I like how it's more lighthearted, also zones, music and architecture are nice
Vanilla - Quests and profs (was messing with UO before, so difference was quite huge).
TBC - Netherstorm! Awesome zone. Introduction of optional (!) daily zones. Solid PvE progression system.
WotLK - Achievement system. Icecrown! LFD (it was decent on release due to community not being in state which it is in now).
Can't mention anything good for Cata and MoP sorry.
Vanilla: Day long Alteracs with Wolfriders, Flightmasters and Elemental
TBC: Raiding, best and most diverse raid content so far.
Wotlk: Death Knight class and Ulduar
Cataclysm: PVE in 4.0 before the "Nerf all and everything" kicked in.
BC : I wasn't there but, the ability to fly
LK : ICC & DK
Cata : Transmogging & LFR
MoP : The insane power of BM Hunter & account wide mounts