[quote=Ellri]
The "purity" of an RP doesn't hinge on the post length or even perfection of the language. It hinges on how well the players mesh. If the players work well together and can adapt to each other easily, then the RP will usually end up being an enjoyable story, no matter about the factors like language, post length or such.Some players are more suited to advanced, while others prefer casual. And of course there's those that like them both.
[/quote]

I have to agree - it's such a personal choice. Like, if you like Rowlings or Gaiman, Melville or Hemmingway, Wynne Jones or Modesett Jr. We gravitate toward those who write in a manner compatible with our own style of sentence, vocab, story, character. That said - I've still had someone I [i]love[/i] to write with, make up a character who just doesn't like/get along with/mesh with a character I've created. Sometimes, it's the lack of "spark" as well, that can kill a story, no matter how many lines you type. 

As for Adv/Cas/even Free - back when I had time to join group RPs, I looked for the depth of world development as well as the possible co-players who cropped up, and most definitely the GM's pattern of story construction-direction. Much in the way I look at singles for a two person RP now, I tried my best to gauge if the story was going to fit [i]me[/i]. If I have a sense of what works with me and what does not, then I can find the story I will work within best, ensuring a greater likelihood that all will be well in the end. Ya ain't got that Chemistry? Nothing is going to drive that bus to its final destination.