I got some playtime with the game yesterday. It's awesome. The graphics are very clean and extremely pleasing to play with. Few games have this quality of art direction, consistency, and pure hard work put into them and I can't believe how good it looks. It /does/ look better than a lot of next gen games on my 50" 1080p set, despite running a lower resolution. Looking good, in this case, seems to have little to do with technical details beyond the vibrant colors and solid 60-per-second framerate.
The sound is amazing. It seems like the directional surround sound is more accurate than in some other games, although Metroid Prime had very accurate directional sound it wasn't used as much in the environments (just during fights). The environments in SMG have waterfalls, cannons shooting at you, and all kinds of other things that make use of the directional sound. The music, however, is completely amazing.
If the music continues throughout the game like it sounds in the first half, it may perhaps be the best soundtrack in any game.
The two-player coop is nice. It allows a couple people to play together, and that's how I like to play games like this, and both can contribute and have fun. You trade the main sticks every other star, as always, but when you're using the secondary ones you can stop enemies, shoot them, gather star pieces just by pointing at them, shake grass and flowers to gather more star pieces, and help mario jump and perform spin attacks. Of course, if you're not careful, you can mess up what the first player is trying to do. But it's pretty easy to be careful, and a lot more fun than it sounds like it should be.
