Polymorph has lots of opportunities to synergize well with competitive questing and roleplaying. Imagine disguising yourself as a chicken or something to see what your buddy is up to, and then ambushing him at the right moment. Of course, once people become familiar with the skills, it'll be less sneaky, but it'd be useful on the first playthrough at least.
Fan of the bard tree (obviously :P). Then alchemist and summoner, and most of the other trees sound meh. Everything has potential, but the likes of unarmed and guardian sound quite difficult to balance/keep interesting.
