Well, there is: One that doesn't express your viewpoint well, for example. It depends on precisely what you are trying to accomplish in writing it, of course.
Mathematical proofs require a lot of creativivty, too. There are many different ways to prove a given theorem, and it takes quite a bit of imagination to come up with one.
I'd say the main difference is emotional. Being creative in the humanities requires a certain emotional madness, which can be especially debilitating. Scientists and mathematicians mostly just need to be obsessive, focused, and good at ferreting out hard to recognize patterns.
Mathematicians typically deal with abstract problems that have no direct relation to human feelings. Novelists, artists, and social scientists typically do the opposite.
