(Who wants to eat only one kind of food?)There are about as many writers out there as there are grains of sand, and each one has his or her own recipe for brilliance. Some say to outline, some say to have a writing niche, some say to write what you know. The roads to success are as varied as the people who write them, but there seems to be one thing a large number of them agree on - that you should stick to one genre when you write.
To many, a romance novelist should always be a romance novelist. After all, that's that person's specialty, so why would they try to branch out? Each genre has its own rules, accepted writing styles, and audience, and it can be hard to move between styles and demographics. Furthermore, your audience is used to you as a horror/sci fi/thriller author, and you risk alienating them by writing something they dislike.
To that I say - horse hockey!
A story is a story, and just because a writer is good at one area, that doesn't mean he or she doesn't have ideas in others. I like to read and write both sci fi and paranormal. Just because I like a good monster book, that doesn't mean I have no interest in politics. Just like I like to read across these genres, I also enjoy writing across them.
Now I know that approaching each genre means altering styles, but I expect this as a reader, so why would I be incapable of it as a writer? RA Salvatore is known mainly for fantasy, but he also wrote he kick-ass novel in the Star Wars universe that lots of folks have taken to. In doing so, he gained new readers, and he drew his older fans to new works. By going across genres, he opened up his world to things most writers never dare to dream about.
I think this whole "you should pick a genre and stick to it" thing is bullshit. If you have a good story in mind, write it down. Don't worry about the gatekeepers of the traditional world - they usually have a greater grasp on bureaucratic rules than they do on what's right. Yes, you have to keep in mind the varying styles that you might need to alter, but that doesn't mean you should pass on a great idea just because it's outside of where you've been pigeonholed.