A lot of writers will not write about anything unless they have actually experienced it themselves even if that means actually getting arrested and put in a jail cell just to experience the sensations etc.. Although this might seem a bit extreme the results can often be gritty and more realistic then the writing of someone who hasn't experienced it. However this isn't always the case especially when it comes to fantasy and science fiction authors, for much of those writing styles a jump of faith is very often needed.
In some ways I write what I know about, my writing tends to lean more towards f/f relationships and BDSM however there is a lot that I don't write about that I have experienced, my faith is one. I'm not sure why I don't write about Wicca and paganism, I have ideas for children books and other such tales but I don't feel the need to write characters that have a particular faith. I guess part of me is worried that I'll write it wrong, as Wicca is such an open faith there are a lot of opinions on many many different aspect's and I know you can't please everyone but I guess I'm worried that by writing something with a pagan theme to it I'll open the faith up more to stereotypes. I would like to write something, but I think it will take time for the words to correctly form in my head.
At the present I'm happy writing about some things I know and not all of them. As for the fantasy and science fiction I write I do enjoy it and hopefully will be doing more in the future possibly even blurring the genre's a bit.