This is probably going sound like an odd answer, but I think the primary thing that makes someone a good RPer is empathy. The ability to put yourself in someone else’s shoes (primarily your character’s) goes a looong way toward the quality of the RP you provide, regardless of your posting style. Writing does, however, play a massive contributing role for me. I like lots of detail and lots of character and story development. Anyone who posts less than 4-500 words in a post loses my interest almost immediately, and my average post is around 6-800 words, ranging up over 1,000 if I’m really inspired. That being said, I find that you can teach people to write well, but empathy isn’t something that can be taught. Some of it is of course innate ability. Some people are just more powerful writers than others, but that doesn’t mean everyone who isn’t a naturally gifted writer isn’t good, it just means they have to work harder at it. Add in a dash of communication, compromise, passion, and a healthy dose of creativity and you have the makings for a great RPer.
I look at roleplaying as a mixture of improvisational acting, storytelling, and writing that kind of gets crammed together, fused and mixed until you find a happy medium.