This opinion is from a person who has RP on MUD, tried RP on WoW
1) People who use rp as an excuse to act like a rude person, people who are mary sue/gary stu who come up with overly used backstories, those who make up backstories that have 0 sence in the lore of the game such as " i'm a orphen 1/2 dragon 1/2 vampire who was the bastard son/daughter of a noble, whos family was killed infron of me", and those who ERP.
3) the type of behaviors i just listed was commen in WoW, most people just sat in taverns and would play their character in conversations only.
Another thing people do is come up with short one time story like plays they rp out with friends
addition: I'm more used to MUD style rp, as its mostly organic. You just simply come up with a personality, a basic backstory and get into the mind of the character your playing, and just play out your character Id love to rp on ff14, but the style I'm used to isn't really done. (from my experience on WoW) And the 2 "rp" servers on ff14 are blocked from character creation.