Name one good thing of each expansion

[Deleted]

BC: Kara <3
Wotlk: Ulduar / Double instant poison weapon swap Assa in T9 and start of T10 <3
Cata: Transmog
MoP: Account wide things and beautiful areas

[broski98]

TBC-flying mounts
WOTLK-battle of the undercity..best quest ever
cata-transmog
MoP-questing....actually having fun questing in pandaria...

[Deleted]

BC - The introduction of Heroics
WOTLK - The Zombie invasion
Cataclysm - if finally ended (the expansion that is)
MOP - Proper Faction reps.

[Vayshan]

TBC: Classic but better.
WotLK: Arthas!
Cata: Firelands.
MoP: Accountwide junk.

[Amsden]

Vanilla: WoW
BC: Flight
Wrath: Achievements
Cata: Transmog
MoP: To early to pick one

[Irraticate]

BC: SSC/TK
Wrath: Phasing
Cata: Revamped zones
MoP: Me gonna Ook you in the dooker!

[Mormolyce]

Just one?

BC: beautiful new zones
Wrath: epic storyline
Cata: old world revamp
MoP: scenarios... no wait pet battles... no wait account wide achievements... argh!

Lots more I wanted to add but could only pick one :S

[Byniri]

Classic: didn't play
BC: didn't play
Wrath: Arthas
Cata: Transmog
MoP: Elite Champion rares

With MoP I could name TONS of other great things.

[Constraint]

Classic - Chaotic yet (to me) fun imbalance, variety.
BC - Flying, of course ^_^ 25s/10s as they should be too, in my opinion (25s RAIDING, 10s casual)
WotLK - Lich King storyline / Achievements
Cata - probably transmog as most have said - I personally strongly disliked the entire expansion
MoP - return to a new continent+the fact that it's not Cata

[Itisamuh]

Vanilla - WoW was new, and thus incredibly epic. With no player expectations yet, everything was just loose and fun.
BC - The Isle of QD is undoubtedly my favorite area of the game, ever. So much epic world PvP. Good lore too.
WotLK - The Lich King and Scourge story is by far the best part of WoW lore. Northrend was awesome, and DKs were a great addition.
Cataclysm - The only thing I really liked about this expansion was transmogrification. The old world was improved, too.
MoP - The Horde vs Alliance war is supposedly back, even though the leveling had nothing to do with it.

[Greel]

BC: Everything.
Wrath: Ulduar hard modes
Cata: Transmog
MoP: Account Wide mounts/achievements

[Draqson]

Burning Crusade - female Bloodelves...
Wrath of the Lich King - LFG-Tool... finding groups was horror, but it also killed community (who needs friends? I go with randoms!)
Cataclysm - Transmogrification (but due to everyone picking that one - fluid questing from 20-60)
Mists of Pandaria - Brewmaster...

[Nekrosislol]

vanilla: New game wow it was awesome
tbc: Man this game gets better and better
wotlk: Man i cant wait to kill that arthas cat
cata: Still lovin the game
mop: too.much.stuff.to.do

[ohshift]

TBC: Flying Mounts and Karazhan
WOTLK: Everything, with the exception of DKs
Cata: Made making gold easier
MOP: Easier heroics

[beanman12345]

BC - flying
WotLK - LFD
Cata - questing
MoP - PANDA'S!!!!!!!!!!!!!!!

[Boathouse]

BC - Flying
WotLK - Northrend in general.
Cata - Hrm...
MoP - THEY'RE TRYING TO GROW OOKING WHEELBARROWS

[steveybaby]

BC: llidan.
WOTLK: LFG, Ulduar hardmodes
Cata: LFR
Mop: 3 hours Queues and the cinematic

[mizeri]

TBC: Flying Mounts and Karazhan
WOTLK: Everything, with the exception of DKs
Cata: Made making gold easier
MOP: Easier heroics

easier heroics? so why bother having heroics why not just make the normal mode a tini bit harder?

[PBitt]

TBC: Karazhan
Wotlk: Achievements
Cata: Transmog
MoP: Challenge Modes

[Aphrel]

TBC: Im torn between Arenas and the raidinstances.. all of the raids was simply awesome.
WOTLK: Achievements! Thou +1 for the questing experience! it was alot more fun then it ever been be4!
CATA: First raiding teir and rag was awesome. Bossfight mechanics has improved greatly over the years!
MOP: Challengemodes!!!