Please try to list only one, two being the max.
And yes, Vanilla is going to be counted as an expansion even if it technically wasn't.
This isn't really a nostalgia thread, hence why I limiting it to two (one preferred). If you can't think of any feature you liked during a certain xpac, just say none. Also, if you want, leave a reason why you liked the features you did.
I'm just curious is all. I like seeing what players like/liked about the game the most.
So to start it off:
Vanilla: 40 mans. I loved these. Unlike 25m and 10m, these actually felt like you were an army pushing through the final outpost of your enemy, well at least they did when people weren't AFK.
TBC: The raids overall. If that's not specific enough then the Sunwell Plateau. Seeing Kil'jaeden almost make it into Azeroth was undoubtedly one of the most epic moments of my WoW career.
WotLK: Well, I've gotta list two for this, LFD and the overall atmosphere. Nothing gets better than snow and undeath.
Cata: The 1-60 Revamp. Although I did love the original zones, I do have to say that I like these newer ones even more.
MoP: Honestly, Farming.