If you're buying a Mario game for the story, you're doing it wrong. Sega has provided ample evidence in the Sonic series as to why Mario games should never have a complex story in the platformers. If you want a Mario game with a story, there are the Paper Mario and Mario & Luigi series. Mario isn't the only game where the story is basically a write-off. I've played a fair amount of Halo 3, but I couldn't tell you the story at all, except that Master Chief fell from the sky and survived.
As for the graphics. They are beautiful. They aren't being judged on the number of polygons in Mario's nosehair, but on how they look artistically, and how they work in the game.
Either way, I do feel Mario Galaxy deserves it's gamerankings score, but in the end, all is opinion. I honestly have no clue how GTA 4 managed to get so high, so if you feel the same about Galaxy, that's cool too.