It's an exceptionally designed platformer, but compared to Super Mario 64 or Super Mario Galaxy, there is a key design philosophy that sadly limits itself more so than not. In other 3D Mario, a gimmick that focuses on Mario's own ability is often never overused--in fact, theses gimmicks are often underused, if anything. But in Sunshine, almost everything is tied to FLUDD. FLUDD itself is a good mechanic--some may even call it exceptional--but that focus limits the level design and ultimately makes the whole game less varied than other 3D Mario games. You don't get the sheer joy of being bombarded with new and interesting things to play with, and while this focus on one good mechanic would've been more than enough for any other 3D platformer, it just isn't good enough for a 3D Mario.

Also, the entire game is basically a water level, so there is that.