I haven't decided what I want to play yet, at all. I know I'm not going for Charr or Norn... and that's about as far as I've gotten. I'm leaning towards Mesmer and Guardian though. Mesmer in particular sounds like a lot of fun.
Mesmer is a ton of fun, though to be honest ArenaNet has been having some balancing trouble with it - nothing that can't be worked out soon, however. Just be aware they're still working on numbers.
Mesmers and Thieves are probably the two most mobile classes, and while Thieves seemed to be more about being evasive and dodging most attacks, Mesmer's are all about deceiving your opponents in PvP. That whole messing with your mind thing isn't just thematic. Between clones, illusions, teleports and invisibility spells, playing a Mesmer is all about not being where your opponent things you are. And then blowing them up.
It's quite fun. Edited, Aug 24th 2012 2:34pm by IDrownFish