Casual can go either way and while it is more coherent, dynamic and with greater depth than Free, I believe that's what most roleplayers are looking for on average because Advanced can be and so often is a devourer of time. You can have the really large, sweeping posts, or the bear minimum paragraphs in the same topic and not really be terribly wrong - barring what your Game Master desires in the initial post of course.

I admit I tend to longer posts despite having reassigned myself to the Casual realm owing to my schedule, but that is by my choice; more often than not because I am trying to acknowledge everything that transpired to some extent and my character's reaction or role in it. This tends to draw it out, but I feel that's fair enough to acknowledge someone else doing something else. That, and there's not excessive details - whole inner monologues, self-discussions, character exposition, plans, et cetera played out - which are only [i]sometimes[/i] appropriate in any scenario... not always.

Going to the other end of the spectrum, Free roleplays tend to be absolute chaos incarnate from what I have experienced and or seen. If you leave for any length of time you've missed ten, maybe twenty posts, and although they may not be supremely pressing or important, you now need to divert yourself into the conversation or action again. The stories are often of questionable quality, with the plot being at most an ill defined paragraph; there's no semblance of remote "balance" either among characters, let alone any sort of character development. Not to say those who enjoy Free roleplays are wrong, it is just a large amount of the material is substandard.

Casual just falls into the right balance, at least the perceived option of it depending upon the individual topic's requirements, or so I believe.