Well basically Mario 64 and Sunshine on the GC, is a collection of levels. In 64 you open doors to open more rooms with paintings in to jump through. In each level you collect stars by doing various platforming tasks. When you have enough stars you can unlock more doors, more levels etc and so on. Basically the aim of the game is to collected all to the stars. I think that's 120 in Mario 64 but I may be wrong.
As for Galaxy we're not really sure yet how it will work. It does look like it might be potentially a bit different. But, it's a safe guess that it will include stars, bosses and unlocking new zones with stars etc just like Mario 64. Spend a little more time with the game though, you may just grow to like it! Also, just on a controversial note - which I will probably get flamed for - I actually PREFER Mario Sunshine on the GC so it might be worth giving that a go too.