Mario on the Wii brings tons of innovation and plenty of fun along with it.
In the 2-player mode, the 2nd player simply controls another star cursor which allows you to collect Star Bits and to help move/stop enemies on the screen. Also if both players press A at the same time Mario will do a Super Jump.
But even though 2-player can be fun, 1-player is just as much fun. With 120 stars on the list to get, there's tons of gameplay and fun ahead of you. The controls are smooth, music can be nostalgic at times but always awesome and fitting to the levels.
In the beginning of the game there is quite a bit of dialogue, but after that there is only a few tutorials left until the game sets you free into the galaxy. If you have not purchased Super Mario Galaxy yet, all I can really ask of you is "Why not?" Seriously, pick it up and enjoy it!