Just put in a couple of hours with Galaxy. I went in expecting mediocrity, and was pretty much blown away. Not only is the whole galaxy concept unique and fresh, but the game plays superbly, and the level design is just great! The game play is similar to Mario 64 and Sunshine at its core; you can jump, triple jump, butt stomp, and now you can spin. You also use the Wiimote as a cursor to gather stars, which are both used to stun enemies and to feed the living stars you encounter, and create new galaxies or planets. I played two-player with my wife; the second player uses a Wiimote and gets their own cursor on the screen to gather and shoot stars. She really enjoyed watching me play, which is QUITE rare. There's just something about this game.
The galaxies part of the game is in the level design. The game, so far, has thankfully been straight-up PLATFORMING. No collect-a-thon crap in sight. Instead, you travel to a galaxy, which is simply a level, and the planets are small mini stages you go through, finding the warp to the next planet. This makes for variety in terrain and enemies all within the same level, although as many of the smaller planets are spherical, you can travel all around them, which can be a bit disorienting or confusing when it comes to taking out enemies upside down. But this is something most will get used to quickly, and end up enjoying. Each time you finish a stage, you get a bigger star, and getting all the stars in a galaxy leads to a boss battle, which then lets you open up more galaxies with the right amount of stars. Along the way, there's also mini-galaxies with challenge levels (including some difficult old-school style platforming, complete with the Super Mario Bros. 3 "air level" BGM playing, and a surfing game where you need to twist your arm to steer; I found this almost impossible)
So far I've only found two power-ups. Rainbow Mario is this game's invincibility power-up, and Bee Mario lets you fly or hover for a short time, although there's no stinger attack and walking in or getting hit with water makes the power-up disappear.
Visually, there's no contest. Saying that this is the best-looking title on the Wii to date really isn't saying much, but Galaxy looks far, far better than any other title on the system. It's pretty much the only "next-gen" title on the Wii, in fact, looking like something beyond what the Game Cube was capable of doing; the worlds and skylines/space in particular just look beautiful. The frame rate stays solid for most of the time, although I did notice a slight drop every once in a while. Galaxy's music is also fantastic; it's fully orchestrated and also has some versions of classic Mario music, particularly from the original Super Mario Bros. and Super Mario Bros. 3.
There is no doubt in my mind that this the best game available on the Wii, and a strong contender for game of the year. Nintendo has really outdone themselves with this one.
I think a lot of people are gonna get sick playing this game.

Yeah, it takes a little getting used to!