I have a 60 Tauren Shaman in Silver Hand (RP server) which I played since release until about february 2006. He's pretty much retired now.
My main now is a (currently 46) Human Paladin in Maelstrom (RP-PvP). I switched because
a) I wanted to see if Paladins are as crappy as most paladin player make them to be (so far I think the class is awesome if played right and I really like it)
b) I wanted to play in a PvP server, which I also really like. The possibility of being ganked at every turn makes the game much, much more exciting.
I miss the Horde, though, but I have to admit Alliance cities are much, much better looking. Horde cities (Orgrimmar specially) look really rushed.