Personally fairly large acts that allow free roaming.. and getting yourself in serious trouble ... can be a lot of fun.
I don't think you will be able to go back to most "act1" locations after act 1 etc, but thats a guess.
With 100+ hrs of fun should be plenty of opportunity for diverse and unique areas.. wilderness.. underdark.. cities.
Someone mentioned "timed" quests... although i would prefer they to be in the minority.. some time quests make sense, e.g. you must get tadpole removed in 7 days or x happens.
