A newer one that I've been feeling lately: People who exclusively play a  single specific character and try to shoehorn them into everything (prior lore from other non-related RPs and media, etc). This is as opposed to taking a character and adjusting/adapting them into the lore of the RP. And definitely goes for canon/IP characters a bit stronger than OCs.  

At best it comes off as having a narrow range of interest; at worst it comes off as creatively stunted or plain lazy. 

Crossovers are the exception to this, of course. 

Otherwise... if you're gonna run mains, at least put the work into making them fit the lore if the RP universe(s) you're placing them in. Or be prepared to be (potentially) limited in your RP selections.