I like the fact that your characters learn by doing. If you want to be a good fighter, then you must fight. Want to be a good blacksmith then you get to the forge and practice.
That, combined with the ability to unlearn skills is what I always liked about UO. My first character started 7 years ago and is completely different now than he was back then. He has the most interesting life story of any character I've had in any MMO game, plus I didn't have to sit there and fabricate it, it came naturally.