i think i just didn't care enough about the characters to give it any higher a rating. for me, the second half was far better than the first but i think that's because i was starting to get to know maria more. the chapters (and book itself) were so short that didion just didn't have enough time to give the content or context that i needed to care about the cast made up entirely of self-destructive rich people. 
it was written well, lots of nice quotes and such but i just wasn't crazy about it. 
though saying that i'd definitely re-read it. i think now i know and understand maria i could appreciate the first half better.