Quality should always be a primary concern. Much can be conveyed in even a handful of sentences with enough flow and context. As long as points are addressed, the categories ultimately fall onto expected capacity.

Free: Some people have the time to knock a dozen posts out per day, regardless of length. With some threads that break my only rule, it might not even matter and may just serve the purpose as a supplementary 'post rate farming ground' for those less inclined to set a scene so much as invest themselves in what their character is doing in the [i][b]immediate[/b][/i] tense.
I've never participated in this site's Free RP and have no right to talk about it, though I respect any measure of investment in the art. We all started somewhere and I'd be lying to say I oozed from the woodworks with wholly comprehensive elaboration and cohesive grammatical enunciation.
I started here back when Quizilla's RP forums were a thing.

Casual: The trenches. 

Advanced: Some people are allowed the time to knock out books. 2-5 pages in a word doc, meticulously looked over with spell checks and oral re-readings to pick apart loose ends to form 'perfection'. I've [i]never[/i] participated in this style since the length could just as easily be supplemented with collaboration to give communication a more real feel to it.
Idk how it works. Its like speculating what goes on in a nebula based on what you think [i]should[/i] be going on.
To each their own.