-It's basically DLC masquerading as a new game at retail price
-Let's add Yoshi, promote the crap out of him, and not use him for 75% of the game
-Recycled "plot" except it's somehow even more stupid and shoddily-pasted together than a typical Mario plot.
-It has so many derivative elements from the first Galaxy, piggybacking its ideas and how ground-breaking it was without offering any significant content that can make it stand on its own.
-Filler stars and filler levels, especially the green stars.
-Arguably the worst final boss in Mario history
-It exposes hypocrisy of several diehard Nintendo fans, when they complain about other companies doing the same exact thing that Galaxy 2 did.
-The excess amount of 2.5d levels in an arguably primarily 3d platformer
-A heavily watered down hub world
I probably missed some but those came to mind. The only pluses? Some levels and the entire soundtrack. The soundtrack is arguably the best thing to come out of it.