Around this time last year, I transferred my main (a character who had been Alliance since his creation) to the Horde to raid with a guild that ended up being an excellent home for me and a great place to raid. I stayed with that guild up until we'd completed heroic Dragon Soul, but recently I transferred to a new guild. And one of the biggest reasons I moved back had nothing to do with either guild (both are fine guilds) or the people in them. My Horde guild was full of people I enjoyed raiding with, cracking wise, doing old content, even making occasional forays into PvP. No, in addition to feeling burned out and needing to raid less, the main reason I transferred back was related to the faction concept in WoW.
Frankly, I wish WoW didn't have factions -- at least, not the big Horde/Alliance split. Because it's made my game playing experience less fun over the years.