The below should in my view reamin the same.

Music(of course!!)
Different races, Different reputations.
Optional quests,freedom of movement i.e you can go anyway and do anything.
I would retain the fact that some skills require you to have certain items. i.e lock picking sill requires a lock pick. In the same way making a potion requires a flask and plants/herbs/mushrooms.
The 4 stats ..i.e strength,agility etc.
The large inventory and encumberances.
The great green outdoors.
The freedom to choose any skill you want.
The abundant item interaction.
The combat/non-combat mode.
head,two ring,amulet,foot gear,belt gear slots should remain the same.
The quests/large map/personality traits/trophies page should remain the same.
Some survivors from <img src="/ubbthreads/images/graemlins/div.gif" alt="" /> 1 should be there as well!!!
One or two towns should remain the same.