Being involved in various groups and sites about writing, I noticed some writers won't stray from their comfort zones, areas which they feel they know the territory and their readers. This is a good ploy in some ways because your readers know you know what you are writing about and know what to expect from you.
I suppose my comfort zone (if I had one) is Chronicles of Mark Johnson, mainly because the world he inhabits is one I create and re-create as I write. Even so, I try not to get too complacent with the stories as I like to keep the few readers I have in suspense, which from the feedback from my 3 or 4 main fans is something I am able to achieve with each book.
If you take a look at my output, you will see an ever-evolving trend of pushing the boundaries of my style; I am striving to reach a new audience and to try and find an area which I fail to please my readers, as yet neither has arrived.