I've been in both categories since 2008, so I'll put it as plainly as I can, whilst still retaining every detail I think is relevant.
Casual is a section dedicated to a more games-like atmosphere. Language comprehension varies from basic to post-secondary though tends to average out around junior high level. Its main strength is the versatility it can bring to the table: You can see anime RP's, fandoms, original content, fantasies, sci-fi, tabletop-inspired, and so on, all playing out side by side, with few to no conflicts whatsoever. Its greatest weakness though is its versatility: It often doesn't know what it
exactly is between the wide range of tastes and literacy levels, and ergo, you see Balkanization terminology pop up from time to time. ex: High Casual, High Free, Low Advanced, etc. (I've seen all three used.) It can be isolating to new people and create unnecessary divisions.
tl;dr
+Fantastic variety.
+Wide range of literacy levels...
+Widely accepting of many kinds of players.
+Moderate pacing.
+Low barrier to entry makes it easily accessible for both the new and the old alike.
...
-Doesn't understand itself more often than not.
-...Wide range of literacy levels...
-Can suffer systemic voluntary Balkanization.
-Moderate pacing is not well suited towards those with busy lives.
-Low barrier to entry often means both GM's and role players have only rudimentary literary knowledge.
Advanced is a section dedicated to a more literary-like atmosphere. Language comprehension tends to fluctuate between junior high to senior high levels. (7th-12th grade), and averages out around high school level. Post-secondary level is surprisingly about as common here as it is in casual from my personal observations. Posts are long and extremely descriptive, which is both a positive and a negative, depending on what you're looking for. There isn't as much variety here as in casual, but it tends to understand itself better: You don't see the Balkanization terminology here in general. Its greatest strength is its connection to having larger posts which are more descriptive, more can be fit into them, which can be a breeding ground for more complex characters and rich dialogue interspersed between metaphor, ideologue, perspective, and emotions. Its greatest weakness is its larger posts: It can drone on about things that ultimately are red herrings, unfired checkhov's guns, or worse still,
nothing at all.
tl;dr
+Greater literary comprehension...
+Characters and plots tend to have greater depth with longer posts...
+Larger posts allow for more content at a slower pace, making it easier for people who have restricted posting schedules to keep up...
+I roleplay here more often than anywhere else because I'm an advanced author!...
+Fewer regulars here than in casual means that people tend to quickly gain their own "crew" to stick with them, which increases survival odds.
-...That is still only in the high school range, but often thinks itself greater.
-...Or, sometimes, they're just writing meaningless tripe that is the equivalent of textual masturbation.
-...At the price of making the barrier to entry higher than in casual, sometimes to the point of scaring people away who would otherwise fit in just fine.
-...A perfect example of the elitism that sometimes still wreaks sufficiently to drop role plays dead straight away in advanced!
-...Can also mean that if you don't like what's currently popular, it may take upwards of
months before you'll see something you would want to play in as a player.
That's 'bout it really.
Oh, wait, one last thing! For people who are scared of advanced!
Casual asks for one paragraph or more, advanced asks for two or more, both ask for basic to intermediate levels of language comprehension. The difference in "skill level" is one paragraph more, so if you're scared to give advanced a try, don't be. Just remember that the atmosphere of casual is more games-like, whilst advanced is more literary-like. This means that casual's characterization tends to be based in action while advanced's characterization tends to be based in dialogue and biography.
It's the difference between an actor and a screen writer. The actor is remembered for what he physically does. The screen writer is remembered for the storyline he wrote, the metaphors, the emotions, the thoughts, and the dialogue. (Though the two do cross realms sometimes. tl;dr: It's complicated but nothing you really need to worry about, it's pretty easy to pick up on without knowing much.
)