Narcissism as a subject
To me it wasn't that Greenberg was unlikable, as that we didn't get almost any perspective on him. We were trapped in his claustrophobic, narcissistic outlook. I could have used answers to questions like, how much did his music mean to him when he was doing it? When did he start his writing-angry-letters campaign -- was that a function of his recent breakdown or an ingrained behavior? Etc. I felt like the movie kept a tight closeup on him and didn't let us see anything else. It felt like a kind of impressionistic filmmaking that American directors are exploring a lot lately - like Sofia Coppola's Somewhere was as drugged-feeling, slow and lost as the main character. It's not entirely bad as a technique, but it leaves so much unsaid that it's like a short story where too many details are withheld.
For a portrait of a difficult personality, I think of a film like In a Lonely Place, with Humphrey Bogart. Bogart plays a screenwriter with a bad temper and a drinking problem. The film establishes that he's also a decent person who comes to the defense of his friends. You get enough information on him that the plot, in which he's accused of murder, is very compelling. In my opinion "Greenberg" has some good scenes and is well acted, but isn't compelling at all because we don't truly understand who this man is. He's almost like a stunt neurotic -- we're meant to laugh at him instead of feel for him.