To me, it’s always that it should be a fantasy, adventure, or mystery RP/SG, of course. Or that I already have a character and story idea that might be fun to play out in the RP, and in this case, I don’t think the genre matters.
Also, the plot. Most importantly, if it’s moving and has a direction to arrive to.
Any idea that seems unique and interesting, where the creator seems to have put thought into their idea. If the plot catches my attention I’m definitely going to check it out and the effort to make it something special on the creator’s part helps me go further to join.