The last game I saw anything like that one was Goldeneye for the N64. Even then I'm not sure if you had to wait until the end.
It wouldn't be a bad idea, but maybe make a cheat mode on the game?
Go into cheat mode, and all missions are no longer avilable and you cant save the game. Then go back out and stop being a noob and carry on with your non-noobish missions. Sounds stupid no?
What pisses me off about Rockstar is their websites for GTA games aren't really launched until maybe 2 months (maybe less) before release date. I would at least like some teaser trailers on the marketplace by May! I know they have a site set-up, but it's just a splash page like many of you already know. I need to see some pictures!