I generally think of Warriors are pretty happy guys, but even they have had their problems. And while most Priests I know tend to actually be pretty cheery about being the first to die when aggro is lost and playing whack-a-mole all the time, they've had their problems as well. And while I do like my Shaman, I can't really defend my class-- we're pretty big whiners (even if it is justified).
So maybe it's true-- Warlocks just might be the happiest class in the game. It's not that there isn't Warlock QQ, it's just that, for most of the game, it's been about 'locks, not by them.