Well, it varies from category to category, obviously (writer, humor, kids, etc.), but the constant is "best". The committee and voters thought this was the best foreign comic published in the US in 2008 (aside from manga, which gets its own category).
I more or less agree: I think it's fantastic, and can't think of any better candidates for the award (though I haven't read any of the other books that were nominated). But I can understand that it's not for everyone. As much respect as I have for, say, Chris Ware (this year's best writer/artist), his comics do nothing for me.