Definitely Los Santos, no question about it. Looks like some of the surrounding area as well, but probably not all of San Andreas. I'd actually expect a map about the size of Liberty City--not too much bigger.
I see a convertible with a working top. Will that be in-game or is it just part of a cutscene?
The lighting and weather effects look great.
No idea which one of those guys is the main character--if any of them are at all. The storyline--so far--doesn't seem much different from GTA4--man running from a dark past looking to start fresh, gets caught up in the same old same old. Here's hoping they do something interesting with it.
I wonder how many of the vehicles used in GTA4 will return. It seems to be also set in present-day, so I'd wager more than a few. Also it seems airplanes are back. Whether we'll get to pilot them, or if they're just set pieces remains to be seen though.