People apparently like the farm a lot. It's not my taste.
to let the list go on:
Challenge mode
Brawlers Guild
crossrealm (actually used a lot, hence popular)
realID
LF(D/R)-tool
BoA gear(Don't think we had that back then but my memory might fail me there)
Most stuff being accountwide
Arguable you can say challenge mode isn't very popular, but it was only intended to be for few people, and blizzard are very happy with how that turned out so far.
Soon to be:
training grounds.
connected realms, hopefully solving the problem of deserted realms.
Flexmode for beerdrinkers.
BoA weapons from raiding.