Better and Worse are truly matters of self opinion. Nobody can tell you how to view yourself, and your own writing, but it can be expressed through their viewpoint. I use these terms, but that only means that in some way I enjoy one particular style over another, that does mean someone is bad, or should be categorized.
To me, Casual and Advanced are more like a genre, than a tested skill level. Casual is exactly as it sounds, with its tweaks. Typically people are more diverse and acceptant here, from my experiences. The standards of writing a bit more lax, and progression tends to happen faster, with variables of course. On the other hand, Advanced is more broad, but in the regards of definition. Here a player could expect a set of rules and expectations that are borderline strict. From what I see, and do, storyline progression happens slower, but the posts are detailed. Also there is a feeling of limitlessness in advanced, for me. I no longer feel constrained by social labels, there's nothing higher than advanced. Sure somebody could create pseudo-labels like "Elitist", or tell you that you're not good enough. But again, that all boils down to personal opinion.
The biggest challenge I see that faces players when attempting to cross that unsteady bridge connecting casual and advanced, is the terminology. In society I was taught that people who were advanced were those in higher courses, special curriculums, at elevated job steps, etc, and these things can sometimes feel so far out of reach that the motivation to apply yourself to it at all can wither away, buried beneath self doubt. That and there is the social group aspect. For example, take high school cliques. A "nerd" does not typically wander over and begin to freely converse, comfortably with the "populars". Typically a person stays within their own group, who they know, people who accept them. We're a species of luxury and comfort, usually, and complacency is something everybody is guilty of, at some point, in some regard.
I remember when I first made the jump. I felt I was a good writer, I had also been told so. I wrote a horror story (two pages) in eighth grade that made my teacher cry. I was still timid, I didn't feel good enough, these writers were my "betters", experienced and savvier in the craft of creative writing. Admittedly my writing was occasionally very far from par, and I didn't get accepted into many. Even then I would be near to last to post in the IC, the feeling of being unworthy sitting heavily on my shoulders. Eventually, I went back to casual. But I did read, I did learn how "advanced" players write, which gave me an expectation. A really, simple, expectation really. Google.
"Define <word>" Is the search I input most often. I try to never use a descriptive word twice in the same post. Using that google search gives a great, brief explanation and a list of suitable synonyms. Then I expanded my mind; instead of thinking:
"There was a wooden table against one wall, the evidence of a prior meal scattered on it. The man walked over to the table and leaned against it, trying to catch his breath."
That doesn't really set the picture, if I was describing a movie scene to someone who was deaf, would that be justifiable? Now I write more along the lines of:
"...A wooden table, crafted from heavier dark woods, was placed snugly against the far side of the cramped, dusty room. Grimy porcelain dishes littered both ends of the long table, a small cracked cup lay upon its side in one of the plush, red velvet chairs. A man of ample weight sauntered over to one edge, bracing his sweaty mitts upon its polished surface as he tried to catch his breath in ragged gasps."
A little description, alright maybe a bit more than that, yet my scene has almost tripled in size. Let me break it down.
"There was a wooden table {Made from what?} against {Synonym?} one wall {Can I set the picture better?}, the evidence of a prior meal {What kind of evidence?} scattered {scattered how?} on it{Can I set the picture better?}. The man {What kind of man? What's he look like?} walked {Synonym?} over to the table{Can I avoid reiterating (Repeating) that it was a table?} and leaned against it, trying to catch his breath {How?}{Can I set the picture better?}."
Anyways, that's my process. I feel Advanced is more about putting in more time and effort, showing that you really care about this story and have an attachment to it. I also tend to stray away from very generic characters, trying to create someone who catches interest, but in a way pleasing to myself to portray. My vocabulary isn't actually very "wide", but I can stop myself and use google to "Define <word>" and find a synonym to avoid restating something excessively. To be hypocritical, because of that my vocabulary has increased substantially since my days of Warcraft and SoTDRP. My newest favorite word is Iniquitous, meaning unfair and morally wrong. I got that from defining "Evil", it's a synonym.
Go google, you beautiful learning device{Googled it} implement you.