The real prize here is Mark Ruffalo. He shows a care-free dude-like attitude, but where most actors could have played this without a soul - Ruffalo gives him many layers (credit to the script and direction as well). Coming from a female writer/director, you might expect this character to be a one-note flake from start to finish, but he is complex and you find yourself rooting for him (at least I did).
Annette Bening and Julianne Moore are exceptional as a lesbian couple with the same problems any married couple would face. I don't know how you could pick one over the other for any award consideration... it's sort of a toss-up with who's better. I liked Moore's character more, but I could see how Bening would be embraced by critics more. She's got a real sharp edge.
The one actor who seemed out of his league here is Josh Hutcherson, who plays the son Laser. He may be a capable actor, but surrounded by so much talent he stood out.
A special mention to Yaya DaCosta, who started her career off as a model, but played a hostess and lover of Ruffalo's Paul in the film. Most of the time the ability doesn't match looks, but she's pretty good. And she's got a unique look too.