A lot of the reviews I've read have been saying that getting missions are far more dynamic now. You don't have to drive to the spot, get a cutscene and do the mission. Now your contacts will call you, let you know they've got work for you and you can either accept on the spot or call them back. It sounded intrusive to me at first, but everyone keeps drilling that it's more liberating than anything.
Also, in regards to repetitive missions, the Rockstar team has put a focus more on storytelling during the missions. So, yeah, we'll get a cutscene at the beginning and the end, but we'll also be treated to the story throughout the mission as well. Missions are said to take 25-45 minutes each (I'm sure there are exceptions) and after 20 or so hours they said they never found themselves doing the same thing.
This is all from IGN's interviews with Rockstar and their hands-on stuff.
Obviously the real star of this game is going to be the online, despite how amazing the single player is. Ever since I first played GTA III my friends and I have said how much fun it'd be if we could all be playing at the same time rather than just handing off the controller every time we were wasted/busted. Free Play will break the world.