I agree with Philip Lopate on the following two things he says a fiction writer needs to have:
Sontag felt the big game was fiction. And that’s where you win the Noble Prize. You don’t win it for writing essays. That’s understandable and that would’ve been great had she been a great fiction writer. Some people can do both, but she lacked a deep sympathy for other people—which is okay if you’re a critic because you don’t have to be that empathetic if you’re a critic, you just have to know what you think about something. And she lacked, for the most part, a sense of humor. It’s hard to be a great novelist without those two things. Somehow she also disdained realism and naturalism for a long time, so that meant she didn’t put that much emphasis into building characters and situations but was much more interested in experimental fiction; when she practiced it, it seemed a little dry. I’m not saying anything that devastating because she was so good an essayist, it’s not a crime not to be a terrific fiction writer also. It’s just that because I love the essay, I regret that she came to put her eggs in another basket.
Empathy is essential to any kind of fiction; a good sense of humor, though essential in any case, would seem to be more necessary to experimental fiction (perhaps because playfulness is experimental literature’s stock-in-trade).
Perhaps this is why very good critics rarely make very good novelists (with James Wood being the first example other than Sontag that pops to mind). Criticism and fiction are both kinds of creative writing, but they are very different kinds of creative writing, and it’s rare to see someone who can truly excel at both. William H. Gass is a good example of someone who has, although reading his criticism (or his fiction) you begin to see why.