Metagamers. These are the folk who walk through an RP and instantly know everything about you and anybody else around you and pretend they've known you for ages and ages and they're very annoying and I need more full stops. I once met this unbelievably annoying Gnome who insisted he'd known my character all his life. O_o I eventually just hid. xD This is as annoying in RPs on MMORPG games as it is on forums, let me tell you.
Mister Lordly Lordus IV. There always seems to be a cretin or two who enters a roleplay and insists he's in charge and demands that everyone respect his authority. As you can imagine, I'll happily run a mile to avoid order-barking clowns like this.
Besides RPs based in MMOs, this can translate to forum RP too, whenever a bad roleplayer decides that their character is the most powerful and influential. X_x "do wat i say or i keel u!!!" "no lol /kick"