Super Mario Galaxy was a great game, and I put it up there as the best platformer I've ever played next to (if we aren't including Mario titles) Psychonauts. The concept is beautiful, the game was beautiful, the physics were beautiful, everything about that game was gorgeous.
Of course, then came along Super Mario Galaxy 2. It was also an extremely gorgeous game, but my question is "Did it really justify a $50 purchase (assuming you got it new) if you adored the first game to death?" My answer would probably be no. It was still very exciting to play, but it felt very similar to the first game, maybe even different in a little bit of a bad way. Unlike the first game, which was designed specifically with a celestial feel (hence the galaxy part), Super Mario Galaxy 2 felt very flat, almost like Super Mario 3D Land (I haven't played the title, so I can't judge). So, a Super Mario Galaxy 3 would leave me a bit uneasy.
Now, if Nintendo was to make Super Mario Galaxy 3 more celestial than just have platforms randomly placed that you can warp to, then I'll be the happy camper I was with the first title. ^^