Since I loved the first season of United States of Tara so much, I was surpirsed to find out that creator/lead writer Diablo Cody wasn't really happy with it, and felt the need to, if not revamp, perhaps tweak the series. Since I thought it was great the way it was, if it ain't broke, why fix it, right?
The main difference, right off the bat, is in Tara. Although the titular character, in Season 1 Tara herself was usually subdued, frazzled, overwhelmed, or embarrassed, and it was the alters who stole the show. It seems like Season 2 finds the show taking great pains to make Tara more interesting, even without her alters. I'm not sure this always works. Like the Bob Newhart of her personalities, Tara is perhaps better off playing straight man and letting T, Buck, Alice, etc play their outrageous archetypes for all they're worth.
The storylines in Season 2 probably succeed about 2/3 of the time. And even when they don't, they're not terrible. Tara's creation of an alter who's a New York psychiatrist is one of the season's best storylines, as well as how it ties into the revelations that finally allow Tara to find the source of her DID. Buck's romance with a barista, Marshall's awkward attempt at heterosexuality, and Kate's friendship with an eccentric artist are all some of the season's other highlights. Charmaine's wedding storyline is probably the weakest, and a waste of Rosemarie DeWitt's acting chops, and the introduction of the Gregson family's gay neighbors added nothing.
The performances are what continue to elevate the series and compensate for any unevenness. Toni Colette continues to knock it out of the park, while playing what is really a series of distinct roles. (She's like a subtler Tracy Ullman.) And the actors who play Marshall and Kate are probably two of the best and most underrated actors on television.