What we feel about the differences between the three categories has more or less been stated by others already.

The most important thing we have to say is that people should not be scared to try a level above where they think they are comfortable. Casual might seem scary at first, but if you avoid the wrong RPs, you'll find that the plot is well worth it, without having the quite-so-impossibly-fast pace of some free RPs.

As advanced goes, this category is probably the one the greatest number of people are scared of. Which is kind of silly. At least where our advanced RP is concerned, the only real requirements are that you can follow the RP's lore, write with decent lingual quality and have fun. Some may write near essay-length, but others can achieve just as much in 3-4 paragraphs. The longest advanced posts typically have multiple authors, and its a well-known fact that a collab can easily get far longer than what most people write alone.

If your English is of relatively good quality (doesn't have to be flawless), don't be scared to try advanced. If it doesn't work out the first time, don't run. Just leave that RP.
Either wait a while, or just jump into another RP, perhaps with a different GM/player group. Perhaps try a different type of character for yourself. Not all characters work for everyone.
If advanced RPs don't work for you after five or more attempts, only then might it be worth considering that advanced really isn't something for you. Not before. There are so many different GM styles that trying any less will not give you anywhere near any idea of what is available out there.

The gist of the above: Don't be afraid to try more than you're initially comfortable with. Most RPers here don't bite. Much.