What happens when you throw in a blender Super Mario Galaxy and New Super Mario Bros Wii?
Most of the people see this game as an improvement of Super Mario Galaxy and base their score solely on this fact, well of course it is an improvement, when new stuff is added to an already great game you can't go wrong. But remember that new stuff doesn't beat the breakthrough of the original game in matter of concept, controls and gameplay.
If you've played Super Mario Galaxy and completed it, twice to get the 121 stars (that means 241 stars in total and 30+ hours of gameplay), you're going to find this new installment of Super Mario Galaxy repetitive and even boring, the old "been there done that".
Yes, I know you get Yoshi, a middle stage checkpoint, new powers, longer levels, a higher difficulty, and a fancy world map but in the end it doesn't make any difference, it's the same game, same plot, same goals, same collectibles, same controls, same everything. And it's not like we've never seen most of this new stuff before, they're all (except for the powers) in the New Super Mario Bros Wii game.
The strong point of Super Mario games isn't exactly the plot, it has been the same for over 25 years, instead they are gameplay, controls and visuals (since the polygonal 3D advent). And they're OK in this game but I can't help feeling being cheated when this game doesn't contribute with anything new to those three characteristics, like every other single Mario game out there.
On the other hand, if you've never played Super Mario Galaxy but you bought Super Mario Galaxy 2, that's another story. However, while reviewing this we can't deny the existence of it predecessor.
Back in the old NES days (1987) there was a Super Mario Bros 2 game (not the Doki Doki Panic version) that was never released in America. It had the same Super Mario Bros engine but was a little more harder and had a few new things, just like this one. Nintendo made the right choice back then, why make the wrong one now?