The Cooler is an acting showcase for the three leads (William H. Macy, Maria Bello, and Alec Baldwin). Each is excellent in their own way: Macy, for the subtle ways this loser character differs from others he has played (for one thing, he gets nekkid), Bello for the way she makes her beauty look natural rather than actorish, and for the way she makes her involvement with Macy's character seem believable, and Baldwin for not so much playing himself as playing the version of Alec Baldwin that we get from the tabloids.
The movie itself isn't much ... it's not really about anything (there are pretensions to young-devour-the-old homilies, but they are never more than pretensions), and the characters are not out of "real life." Which makes the performances of Macy and Bello so remarkable: while we're watching, the characters do seem real, not just fictional constructs. (Baldwin's character is always fictional, but as he plays it, that seems appropriate.)
Baldwin seems to have the inside track on an Oscar nod, although Bello deserves one, as well. Of course, Macy deserves one, but then, he always does, even if he's only ever gotten one nomination.