The fact that they seem to have captured the feel of being a real person. If you are on a catwalk and cut a corner your character actually does the extra hop to make it, etc. All the small movements that automatically happen just like all the movements yo do in real life withot thinking about it.
I know that sounds lame, but I can't quite describe it other than it truly rocks!
I have been a practitioner of various forms of medieval European swordplay since 1984, and the old Jedi Knight drove me nuts because there were so many things that you could do in real life that you couldn't in game. I always told myself "oh well, some things you just can't capture on a PC..." Well so far, I think I have been shown differently!