Before you guys all jump on me, hear me out.
First, I'm not knocking the current console versions of the game in any way. The game is brilliant as is, but after playing through the story and messing around with the online it occurred to me how amazing the potential here is.
-The RPG elements
-Far better graphical detail, draw distance, AA, and particle effects
-The modding community can do some pretty amazing stuff
-Pinpoint accuracy with the mouse. Shooting won't just be autoaim --> shoot. There's going to be better flow to the combat.
The graphical style and visual difference is going to be crazy. It's going to be a much more refined experience.
edit: assuming that they don't mess it up while porting it