Outer Space becomes Mario's playground in this wondrous platform creation from Nintendo...
First, the plot is the most obvious weakness of Mario Galaxy. Simply put there really isn't a plot. Sure, there's the typical kidnapping, attempt to rule the universe stuff, but nothing really intellectually stimulating. The absence of any plot leaves me wandering...was this intentional? One could hypothesize that perhaps Nintendo, wishing to please a wider audience, sought to minimize the plot in a way that would avoid alienating anyone. Too complicated a plot may leave younger players bewildered, and an overly amateur plot may send the older crowd running. Instead it appears they opted for a simple plot that was non-invasive to the actual gameplay. This concept works fairly well, but recall Mario and Luigi: Superstar Saga was quite plot heavy (and an RPG to boot) and successful. Overall I think most would agree that a lack of any plot will remain Super Mario Galaxies primary (if only) weakness.
The second main problem faced in this game centers around depth perception. This is due, in part, to the camera. The controls are affected by the position of the camera. Restrictions on how the camera can be positioned can lead to some difficult situations. Thankfully, this is not a typcial problem and can usually be adjusted for once you get used to how the game works.
My last thing I want to point out is the omission of several key Mario characters. Characters like Daisy, Wario, WaLuigi, Birdo, Yoshi, and so on have been for the most part left out of this game. Why? Particularly Yoshi. The new characters are great, but it wouldn't hurt to have the old kingdoms included with part of the universe.
These minor issues aside it's hard to really complain about a game this good. It's fun, and isn't that what it's all about?