What I would absolutely hate is Slo-mo... I've used VATS in FO3 so much that I get sick to my stomach every time a game goes into slo-mo... seriously it's cheesy, annoying, and a waste of time. Sure it's awesome the first 25 times or so, but when you expect to play a game for 100+ hours, it really adds up.
EDIT: oh and I also hope that finishing moves don't occur when your opponent has 1 hp left (or any amount of HP that can be subtracted with a single swing)... it would really seem unnecessary and over the top. Also, like i said before, it would be a waste of time.