1. Players who want to metagame their careers upon their characters.
"Dude, I'm an MP--"
"But you're playing an elven magic-user..."
"Regardless, I deal with yahoos like me every day: the town constable would never say that! You've gotta change the roll."
2. Players who try to hoard every item they see, even when their character can't use it.
"John, I'm not gonna let you take the scroll--your dwarf can't even read it."
"I'll sell 'em later! It's my loot!"
"John, Mike's character will die unless Karen uses that scroll."
"Hey, Karen--wanna buy this scroll I found?"