Hmmm... here are some I can think of right away...
1) More weapons/ammos/items in general
2) Either better AI on party members or better control over them (But not in the same way as in Fallout: Tactics)
3) Very big world to travel, and also lots of different cities and towns to visit.
4) More variety to what NPCs looks like, it gets quite boring after some time when there are 5-10 different way character can look...
5) More mutated animals and monsters to kill
and finally, but definately not least, as so many can agree with me...
6)
Not so damn buggy!!!!