I had to vote None because I feel this way too. There were too many "fun" announcements to just choose one.

Even something like the Pet Battle System, which the naysayers are already bashing as "LOLOLPOKEMONOLOLOLLOL FAILLLL", which is billed as content for non-raiders and non-pvpers, has be excited as a raider. Right now in live, I generally don't log into the game outside of raids...there's nothing to do. But the PBS looks to have enough content in it to whet the appetite of many players who just want to chill for a bit and have some good old-fashioned fun. It doesn't seem to have the hectic pacing or the adrenaline rush of a heroic progress raid or a PvE scenario (I assume the scenarios will be tightly tuned, especially since they know what everyone's gear level will be), but that's the beauty of it. You get to chill for a bit and still have something to do.

Combine that with all the other parts of the expansion (even the art style which is breathtaking), and it's really cruel to ask us to pick only one. :P