Worth noting that the "Welcome to Reading Excuses" guidelines do address this (this line is kind of a holdover from the group's very early days, but it was a deliberate decision at the time):
I'm in favour of the "all genres." One, because that way we're not inadvertently excluding genres with which there's a lot of SFF crossover (say, horror or paranormal romance) but also, I am personally a big proponent of critiquing across genres. As an undergraduate I spent a lot of team in creative writing classes where I was one of the few SFF writers, and seeing the variety of works across different genres was a real joy, as well as a good learning experience.
That being said, aeromancer's also quite right. Since we're hosted on a Sanderson fan forum, the bulk of writers we're going to get are inevitably going to be SFF.