All the women (besides Anna played by the timeless Blythe Danner) are caricatures. Lisa's just a bitch on wheels, and Jenna's knife-wielding outbursts are all too melodramatic and one-note (not a fault of Jacinda Barrett's per se). She is described as being "like a guy" (what, like those sporty-sexy lesbians who will marry your boyfriends?), but never does that actually come through in her actions. If pregnancy hormones are to blame, there should nevertheless be layers to the outbursts.
The male characters are all interesting: from sex-god Kenny to hapless schmuck Izzy. I especially dig Casey Affleck's Chris, stuck in an unhappy marriage and amateur fatherhood. The entire sequence of him having to keep Michael's secret about cheating on Jenna but not wanting to lie is painfully awkward, thus funny.
I'd be more okay with this movie if it just stuck with the thirty-somethings. Michael's affair with Kim provides an inciting incident, but nothing really happens besides kissing in the rain and a mix CD (come on!). Kim is written to be so twee and lame, and Rachel Bilson bats her doe-eyes through it to no great effect.