I played the thief pretty extensively in bw2. Overall the survivability is excellent, you have tons of stealths, and great mobility. The damage also seems pretty solid. That said what I dislike is that the ability que can mess you up if you double tap a key really badly. Other classes also have the ability que to deal with, but since their skills are cooldown limited the skill only fires one time, as opposed to multiple times. This is a problem for me since I spam keys pretty hard, and trying to limit myself to precisely one key press is difficult. My other issue is that I sometimes fell off of structures due to a the repositioning nature of some of the dagger dagger skills, ie; when you leaping death blossom someone you jump behind them... and if you aren't careful fall off the ledge. Finally, I feel like the other classes have an advantage in the weapon swap department; when they switch weapons they have an entirely fresh bar of skills to use, the thief though doesnt automatically get a full initiative bar when swapping, so they cant use all of the skills on the other weapon bar.
Overall I think it's going to end up being a pretty niche class with super high skillcap that really demands alot of precision from the player. Not necessarily a bad thing, just something to consider.