I believe Josh also made one for metal music, although I don't understand why the pop/rock couldn't be used for that.
I wouldn't want to wade through loads of pop-related posts when I'm only interested in metal.
Another forum that I read groups melodic rock and hard rock together, but most of the discussion is about the former, thereby making it harder to find the hard rock discussion.
And a book forum groups Fantasy and Horror together. Most of the discussion is about Fantasy, but I'm only interested in Horror.