I don't know. Galaxy seemed relatively forgettable to me too, at first. Probably because the galaxies seemed spread so thin. Of course, my Luigi playthrough put that game in an entirely different light. Suddenly it became much more memorable. The levels, the soundtrack, everything just clicked.
Honestly, Galaxy 2, as I'm making my way through the green stars, feels the exact same way. The first time through it seemed forgettable in a bunch of ways. I couldn't remember the galaxies themselves, my memory of the music was a bit fuzzy, and there was no way I remembered every galaxy's name. Now I can remember pretty much every galaxy, theme, and anything else. These games are built to be played till 50% or less by certain audiences, and 100% by other audiences (the hardcore, pretty much), and for the latter, a lot of the game's values are going to shine through via constant exposure to each particular galaxy across many hours of play/replay. It's true that this sequel feels even more spread thin than its predecessor, but that's not necessarily a con. I guess it just depends on your expectations.
Oh, and the soundtrack reception is befuddling. Some people say it's absolutely outstanding, other say it doesn't even come close to the first game. I think it's fair to agree that the soundtrack is just "good." Like the first Galaxy, it has its forgettable tunes, but there's also some pretty great stuff in there. Both Sky Station themes stand out, as does the Yoshi Star (being catchy and all) and Fluffy Bluff (AKA Gusty Garden 2) ones. The speed run/Hurry! theme is better than a lot of themes in either Mario Galaxy game; really intense and awesome. Then there's the more subtle gems, like the Rightside Down Galaxy and Sweet Mystery Galaxy themes.
Some people were immediately astounded or immediately underwhelmed by Galaxy 2. Same for its predecessor. I, personally, think both games are "growers" that get better as you play them more and as you reflect upon them more.