[quote=@Hank]
So the consensus seems to be that people are drawn to the middle-ground, and either extreme of the spectrum scares them off (either by being too shitty or too demanding). Fair enough. 
[/quote]

I think a large portion of RPs in the Casual section could be classified as Advanced not only in terms of length but also story and character development. Although, as others have said, there isn't the pressure of having to worry about whether or not your post qualifies as "Advanced" and if you're having a hard time you can get away with short posts and no one will think anything of it. GM's will ask for "high casual" and usually they end up with mostly Advanced level writers, at least according to the Guild's standards:

[quote=A New User's Guide to the Guild]- Advanced Roleplay - Elevated standards. Advanced RP focuses on longer posts that include character development and elaborate plot development. Suggested minimum of 2-3 paragraphs per post.[/quote]

Most of the RPs I join are in casual but this is a perfect description of the GM's standards and how they usually turn out.