I've been kicking around the idea of trying out PvP, but I don't want to just jump in without knowing if it is in a format I'd enjoy. I've PvP'd a lot in the past, most notably in Diablo II--mainly low level dueling.

From what I've seen, there seems to be plenty of lower level dueling out there, but what exactly happens? Do you join a game, and then instantly it's everyone for themselves unless you state otherwise, or is there some way to keep from being attacked to observe if you choose to?

Really, I'm basically looking for a run down on exactly what goes on in a good PvP game.