Using official screenshots and stills from the game's trailers, a Polish fan site has done a cracking job comparing the San Andreas of 2004 to the one we'll be getting our hands on in 2013's Grand Theft Auto V.
As you can see there are... more trees. And shinier cars. But one thing I will miss is that old sky. As an avid supporter of the Blue Sky in Games campaign, Rockstar's old ones were a lot more fun to drive around under than their murkier, more contemporary efforts.