I really enjoy role playing which is why I run three role-play guilds. At times I start wanting to join more role-plays. When I do start looking for other role-plays to join there are a few things that I do normally want to join they are:
Harry Potter - Don't know why it just always seems fun and I never found one I liked before
S/M - I don't know why but I do enjoy playing a slave. However I don't play masters ever.
Medieval Fantasy - This one a big one. I just love this topic, it doesn't matter how real or unreal the role-play is.
A mixer of these topics are good as well however some might not make much since together.