I agree that Toy Story 3 was really sappy. The first two were, as well, though, so maybe he's just good at mimicking the style of a given franchise. I haven't seen Little Miss Sunshine, but its reputation inspires a little more confidence, I guess.
I don't see why people want Brad Bird for the role. He's only made one live action feature (Mission Impossible 4, a well-made but ultimately big/dumb/loud action movie) and his animations range from pretty good but sorta big/dumb/loud and comedic (The Incredibles) to Ratatouille, which seemed like a really bland approach to possibly interesting subject matter.
I think Star Wars is best when it's dark, emotionally poignant, and there's some complexity to the characters. aka The Empire Strikes Back. Brad Bird's competent but I'd much rather have somebody with a track record of making films like this. Francis Ford Coppola, Guillermo Del Toro, Neill Blomkamp, folks like that.