Liked Onderon, wished there had been a little more interaction with all the citizens and the people of Iziz, maybe a bit more with the Beast Riders too. Now I think about it, i would've preferred it if it was a little more bustle, kinda like Taris' upper city, where everyone was going about their business, but there was some people you could stop and chat to.
There didn't really seem like there was much to do on Onderon either really, a main quest, and a side quest of a murder investigation, that seemed about it to me.
Another thing that really annoyed me about it, is that you have to keep running back and forth between areas, when it could have been so much easier if you still had the transit option like on K1, but thats just general K2 anyway.
/rant over