Other than the apparent lack of a story, I thought this game was the epitome of what a platformer should be. Sure, it wasn't all that challenging most of the time, but playing through the gorgeous and intuitively designed levels was one of the best gaming experiences I've had, personally.
I think, with a lot of gamers, nostalgia is a compulsion that deters fair judgment. Yeah, I'm sure you all like Twilight Princess, but your love of Ocarina of Time may distort how good it really is, as you blow the difference way out of proportion. Maybe you have a good reason but I hardly ever see that being the case, as the rationale of most people is narrowed down to the fact that they were younger and generally were more immersed and far less judgmental; they saw a thing for what it was. Now, however, it seems that new games in ongoing franchises are always going to be openly subject to the criticism of minor differences with its predecessors. Mario has always followed the same formula, and I think, in reality, that Galaxy has capitalized on all the best elements of Mario platforming that have been introduced.
Seriously, this game was gorgeous, to begin with, with an original, fitting score that - while it based itself on preceding themes - captured the atmosphere of each level very well. Best of all, playing the game was a matter of applying a single algorithm of playing to a convoluted playing environment. That composure never seems to go well otherwise, but with Galaxy, it really worked.
I'm not demanding anything of you people, but seriously, this game is truly much better than you critique it as being. It's a marvel among the mundane, and it deserves to be appreciated for all the things it got right; not undermined for all the minor details that fans seem to magnify too much.
EDIT: You pretty much get the whole experience with just Mario, though. Honestly, not much changes when you use Luigi, and in fact, the game ends up making very little sense that way. The only thing it really accomplishes is letting you use Luigi's insane jumps and net you the 121st Star.