So, I'm one of those guys who softmodded his Wii for homebrew use, so I managed to get a copy of the SMG2 ISO a day early and give it a go.
Early impressions:
1) It very much still is Mario Galaxy. It's quite evident that they used the exact same framework (Not that this is a BAD thing).
2) The battles with Bowser don't seem any more difficult than they were in SMG1. Perhaps that will change in the future...
3) Really liking these new mushrooms! The cloud suit is my favourite thus far.
4) There's still an abundance of extra lives everywhere, but considering how frequently I'm losing my lives, it's a justified abundance.
5) The slide course on Tree Trunk Galaxy is notoriously difficult. You had better have some good reflexes if you want to get through it alive.
6) Rosalina only appears for the Super Guide. I don't think she's in the game elsewhere.
7) Yoshi is pretty damn fun to play as. You can still jump off him at will (ala SMW) for a little extra boost, too. I'm thoroughly enjoying levels where he's playable
8) I'm loving the little references Nintendo is hiding here and there...there's a section in one of the levels in World 3 that has 2 sections that are so reminiscent of old Mario Party minigames (Bumper balls and Hexagon Heat) that was pretty neat. I'm sure more references are to come...
9) There's a sunshine level. I'm not gunna say what it is, for the sake of spoilers.