Super Mario Galaxy is one of the best nintendo games ever and arguably the best platformer ever made.
The way it is different can be said to be good or bad depending on your tastes. Galaxy is much more leanier. In Galaxy you dont get the freedom you get in 64. You cant get the stars in a level in any order any more and there's a much , much smaller emphasis on exploration. This doesn't make the level designs any less brilliant. The game is undeniably more fun than mario 64 and the camera and controls are far superior. You do have a hidden star in most levels that require you to explore a bit and go off the main path of the level, but even these hidden stars dont require nearly as much exploration as you had in mario 64.
Enough about mario 64 now. Super Mario Galaxy is incredibly fun and has quite a decent length, whether you plan on going for all 120 stars or not. The use of gravity makes some levels look and feel insane. Insanely good though. You might expect the controls to not work so well when your walking upsidedown, but thats not the case at all. Everything feels completely natural no matter where you are or what direction you want to go.
The boss fights for the most part feel really good, although on the whole they're a little easy. Some of them can be challenging at first but once you figure out what you need to do, its easy as they normally only require 3 hits till they're dead.
I love the idea of prankster comets. Prankster comets make you replay a level with an added twist. A speedy comet makes you have to beat the level against the clock. Daredevil comets make you do something in a level with only 1 bit of healt, normally a boss fight except one level where you have to do an entire level with only 1 health. Fast foe comets make all enemies move really fast. Some of these can be quite challenging and are really fun.
The only really negative things I can say about this game is the overall easy difficulty. People have said its too easy to get to 60 stars to complete the main game. I'd agree with that. I'd also add to that by saying getting all 120 stars in way too easy. It was more challenging for me to complete mario 64's main game than it was to complete galaxy with 120.
Other than that this game is almost perfect. This is definitely the best mario game I've ever played and probably the best platformer I've ever played. To me this is the second best game on the Wii net to Twilight Princess. Whether you prefer Galaxy or TP is down to whether you prefer the platformer or the action adventure. TP is the best action adventure I've ever played and Galaxy is the best platformer I've ever played. I gave TP a perfect 10. The main thing keeping this from getting a perfect 10 is it being too easy. If you own a wii and you like platformers even just a little, you need to get SMG. Why wouldn't you want to own the game of the year?