I'm going to wait a bit and read some insights after the launch, figure out if my PC can run it :P. If not, I may get it on PS3, but I'm not sure. I'll definitely lay my hands on it sometimes after the launch and play the shit out of it.
I want improved multiplayer. Just yesterday I managed to figure my way into GFWL for GTA IV and TBoGT and TLaD and played some free-roam. It's so damn fun, I can't wait to see what they have in-house for it in GTA V.
Favorite character - Franklin. C.J. was pretty ok in San Andreas, a new black protagonist is really welcome from the stereotypical Niko (I'm an Eastern-European and it's funny to see how off the record they've done him, but I get that GTA's aim is to prove a point by throw a shit ton of stereotypes at the audience).
What I like the most about the game... Hmm. I don't know. I have high hopes for the shooting. The heist was my favorite part in GTA IV, so their comeback is great. Car customization was ok in San Andreas, so I look forward to that as well. Too many things to choose :P.