Characterize Nick Bottom. What aspects of his behavior and personality make him comic?

While other characters in A Midsummer Night's Dream are comic in more subtle ways (like Puck), Nick Bottom is a very overtly comic character. The humor comes from the way his beliefs about himself contrast so strongly with reality—in ways that are very obvious to the other characters and to the audience.
He believes himself to be an incredible actor who would be perfect in any part of the production of Pyramus and Thisbe, but his speeches are melodramatic and contain many rhetorical and grammatical mistakes; it is obvious to everyone else that he is a very poor actor whose peers are not pleased with him. He believes it is absolutely possible that the fairy queen Titania could fall in love with him, when it is clear to everyone else that he is not deserving of this. This contrast is made physical and even more obvious to the audience when his head is turned into that of an ass, and he has no idea.
The irony of the contrast between Bottom's beliefs and his reality is comedic, and it also allows for the audience to be "in on the joke."