I never played Skate 2, but I own Skate 1. I just fooled around with it basically, never really completed much of the story mode.
The new one is really focused on online multiplayer. You can have your own Skate Team now with, I think, up to 8 guys (maybe 6). You can challenge other Skate Teams, just skate around for fun, or play games against your own friends.
Played it for a good 4 hours last night, fun game. If you liked Skate 2, then you'll like Skate 3 (in natey2k4's case, if you like Skate 1, you'll like Skate 2 )
I've enjoyed the updated Hall of Meat stuff in the game, being able to aim my doomed body as I throw it off the top of a skyscraper, only to Judo Kick some unsuspecting pedestrian is just too much fun.