Gakked from Ari

Name a fandom and I'll provide five reasons why the character I have a crush on/find attractive in that fandom would be a bad boyfriend or girlfriend in real life.

That sounds a lot of fun. For bigger fandoms (Buffy/Angel or Who/Torchwood/SJA), feel free to restrict it in some way (e.g. "vampires" or "Slayers")--if there's no one I fancy within that class, I'll just say so.