The unusualness of LW may be self-reinforcing. You start a forum, or a blog, and can count on people arriving to it knowing more or less what to expect based on other forums or blogs they've read, as appropriate. As a weird hybrid... honestly, I've was an Overcoming Bias reader before LW existed, and I still don't feel totally comfortable with the format. So that's a disincentive to create more weird hybrids.