I Have No Son: Gets kicked out of her house by her mother and told she isn't welcome there anymore after her marriage is revealed. Given how her home life was characterized by intense verbal and emotional abuse, it was probably the nicest thing she did for Taylor, albeit unintentionally.
I Just Want to Be Loved: She will relentlessly pursue anyone who shows her even the slightest amount of affection due to growing up with a heartless, emotionally abusive mother. She at least seems aware of it though.
The Cast Showoff: Sandy's oft-lampooned fondness for show tunes ends up hijacking a season 2 episode when he surprises Kirsten with a set of love songs on their anniversary. Peter Gallagher has a great voice and it is pretty squee-worthy.
Blonde Republican Sex Kitten: At one point in the show Sandy mentions to another character that Kirsten is a political conservative. There are also a few points in the show where Kirsten criticizes Sandy's "hippie parenting."
Defrosting Ice Queen: Julie Cooper very gradually becomes more sympathetic as the show goes on. By the final season, she had shed much of the bitchiness that defined her in the show's first few seasons.
Nice Girl: Kaitlin has a big heart, and though she doesn't always say it the right way, she cares about people. She once threw a party solely so two girls that she barely knew could attend because they hadn't been invited to the Alpha Bitch's party.
Hidden Heart of Gold/Hidden Depths: Mostly in season 4, but he shows signs of this even in season 3: he was really sad about Johnny's death and deep down, he really cared about his relationship with Marissa, albeit in a dysfunctional way.
Divergent Character Evolution: Very early on she and Summer were distingushable only by hair colour, both being vapid, self absorbed party girls. This is actually Lampshaded by the show in the "The Chrismukk-huh?" where see a universe where Summer continued to be friends with Holly rather than going through Character Development. It is not a pretty picture.
Took a Level in Dumbass: While she never displayed any particular intelligence in her early appearances she wasn't noticably dumb either. When The Bus Came Back however she was firmly in Brainless Beauty territory and there are a lot of jokes about her airheadedness ("Who says I can't have a baby and a six pack?").
Your Cheating Heart: Slept with her best friend's boyfriend. In two different realities no less.