Like what? I would consider the story of Mario Galaxy to be far from one of my favorite moments. While the story was good by Mario standards it was still very basic.
I guess I'd better back up some of my statement...
While the story in Galaxy wasn't really my favorite part of the game, it was still well-done, for a Mario game. It was completely optional, as Koizumi intended, but Koizumi wanted to do more with it. Miyamoto didn't let him. Whether this is good or bad, there is no real answer. I think Miyamoto gives great guidence to his development studio(s), but I find the whole "sneaking it past Shiggy" to be a bit...off-putting.
Yet that's what I personally think. I respect Shiggy just about as much as the next guy, but considering Yoshiaki Koizumi is the guy who made up the entire Zelda mythos in Link To The Past, and was the NPC/Story Director for Link's Awakening, Ocarina of Time and Majora's Mask (unsure of Wind Waker), if we saw him have more story control, we would get something superb.
Unless you are willing to call most sequels out there "expansion packs" then it is asinine to call SMG2 that, even if Miyamoto's statement was true, the game is well over 90% new, that is pretty fancy expansion pack if that is the case.
Yoshi added into SMG2 is basically 120%.
Yet I still think Galaxy 2 won't make much of a mark on million copy sales as NSMBWii will.