What was your favorite feature of each expansion?

[Dragore]

Vanilla: everything was brand new
TBC: shaman/pally playable for both factions
Wrath: LFD
Cata: LFG
MoP: Flex

[Deleted]

Vanilla: Didn't played Vanilla a lot, I only played it a bit on my brother's account, but i guess vanilla was just here to set the basements because that's pure crap compared to what we have now.
TBC: Arena, heroic dungeons, raids no longer being gear checking walls, that's where the game really started.
WotLK: Heroic raid bosses, Death Knights even though it started off badly.
Cata: Didn't played Cata past the first tier, but there's one awesome thing that came with Cataclysm : goblins
MoP: Erh, kinda feels as a generic expansion, which is very good though but there's nothing especial i noticed.

[Seefer]

Vanilla - All the exploration and non-linear questing

TBC - Raids/Heroic dungeons

WOTLK - Environment

Cata - Uh.........Transmog, everything else was shit

MoP - I'd have to go with Cloud Serpents, they are kinda cool.

[reckoner04]

Vanilla: The 'rawness' of all the systems and how the world worked, if that makes any sense. The features weren't super sophisticated (just compare the LFR loot system to back then...) and the world wasn't super streamlined back then.

TBC: Heroic dungeons or Arenas or Outland (sorry for naming 3 things!). Outland was just this completely new and mysterious landmass and I spent way too much on Heroics and Arenas. I enjoyed them a lot.

WotLK: Probably the continent and how raid pugging worked. Not really any feature, but to me the game felt very mature at that moment already.

Cata: Transmog quite clearly. I can't wait for them to implement a proper UI and collection system for it.

MoP: No feature in particular, I also didn't play a lot. It was basically just more of all the good stuff I like already. I would like to say Challenge Modes, but I never got to play them (never found a group because the players are just after rewards ofc).

[Zyrinx]

Hmm well I started (late) BC...

BC - Flying I guess. I was clueless back then.
WotLK - LFD, DKs
Cata - XMog, without which I'd have surely quit by now. Also DWARF SHAMAN.
MOP - This one's hard. Never really connected with the Monk class though I have one at 90. I'm still glad they did it so I'll say the new class. Oh and a small thing but removing the stupid "raid Group" requirement to get into old raids.

[hrugner]

Vanilla: open explorable world
TBC: raid structure
WotLK: world design
Cata: experimentation with new concepts (redoing old world, underwater zone, putting flight everywhere)
MoP: Nothing really stands out. The Klaxxi?

[Failmuffins]

Vanilla: Didn't play! - I only started playing WoW at the beginning of TBC, so I never really got to experience most of the features that Vanilla offered. That said, I did find the massive world quite breathtaking.

TBC: Daily Quests - While I do feel that they got a little out of hand during the opening stages of MoP, I've always enjoyed the sense of direction and 'things to do' that dailies provide. I usually prefer structured content over free-flowing design.

WotLK: Wintergrasp - How I miss the good old days of Wintergrasp. I loved the implimentation of this zone - from a rewarding yet dangerous farming area during non-battle hours, to the closest thing to a large-scale siege that I feel we've experienced in WoW. One of my greatest laments, and why I can't be bothered to PvP in MoP, is the lack of any sort of zone similar to Wintergrasp or TB. Can't wait for the new WoD one though!

Cata: Transmog - Its lovely to no longer have to worry about looking like I had a major wardrobe malfunction, and to have the option to bypass tier sets that I'm not too fond of. If only I had Transmog available back when I was wearing those ridiculously oversized water fountain priest shoulders during the first tier of Cata!

MoP: Item Upgrades, Farm - Item upgrading with valour is a great feature in my opinion. So many times in the past, I'd simply run out of things to buy with valour points, and thus never feel incentive to work my way to the valour cap. Given the good number of options we have to accumulate valour at this stage in the expansion, I find its a pleasant and enjoyable experience to farm some valour and marginally improve my gear every week.

I enjoy the farm because its really the first thing in the game, outside of my own character, that I feel is 'mine'. Yes, it may be instanced with countless other players, but even that illusion of ownership is welcome in my opinion. The fact that through a little daily effort I can have a steady stream of mats is wonderful too. =)

[Deleted]

vanilla: pretty much everything.
tbc: arenas
wotlk: wintergrasp, achies?
cata: arena queue changes, transmog
mop: huh... does the pvp trinket bonus count?

[Fullmetal89]

Vanilla- World PvP was fantastic, truly memorable back in the day when you could get 1 shot by mage/rogue/hunter.
BC- Flying mounts and more accessible raiding, early arenas were fun until tweaks started to make matches last hours.
Wotlk- Raiding, I never really got truly into raiding till late BC but Wotlk was what really brought me into PvE. I loved every raid, yes including ToC; the raids where fun and the mechanics where hard when they had to be like LK/Yogg. Instead of how they are in MoP/BC (especially BC) were certain end raid bosses are easier than some of the other bosses in that same raid. Also LFD was pretty great for those of us that were tired of waiting hours on trade to make a pug.
Cata- Friendliest expansion for altoholics, I loved the leveling experience so much I even got my DK to 85! A class I normally couldn't play for more than an hour at a time. Edit: Forgot to add Transmog, this was the reason I stuck through Cataclysm. Greatest feature introduced into the game since Flying imo.
MoP- Race/Pandaria and especially the Mantid were my favorite part of the expansion as for feature Flex raiding was pretty darn cool.