I haven't read most of this thread because reasons so I may be saying things others have already gone over multiple times. Just a fair warning before you get to the body of my needlessly long post.
My definition of elitism is pretty simple and comes in two forms. Anything that places a higher value on a person/group based on a limited attribute or set of attributes is elitist. Conversely, anything that discriminates against a person/group based on a limited attribute or set of attributes is elitist. These are essentially the same exact thing wherein the difference is the perspective and the kind of language used, but some people call them distinctly different things nonetheless. Some say elitism is only the first kind and that the second kind is just called discrimination. I'm of the opinion that they're all part of the same continuum. As an example of my definition, racism is elitism based on race; anything negative said about a race, including the lovely variety of racial slurs we have, is elitism of the "discrimination" kind; things like saying Asian people are generally smarter than other racial groups is elitism of the "higher value" kind.
To apply this to RPG I'll break it down by section and give an example of the two different versions of elitism that exist therein. I'll call them Type 1 (higher value) and Type 2 (discriminatory) from now on to make it less cluttered.
Type 1: "We're better than Casual and Free players because we write longer posts and have better grammar, plots, characters, character development, etc."
Type 2: "Free and Casual players are awful because they write like shit and have a bunch of vampires and high schools and anime trash all over the place."
Type 1: "We're better than Free players because we write decently and post full paragraphs at a minimum." "We're better than Advanced players because we have all the same writing quality without the ridiculously huge posts."
Type 2: "Free players are awful because they post one liners and can't write worth a damn." "Advanced players are awful because they're all a bunch of egotistical elitists who pack tons of fluff into their posts like it's some kind of contest for who can have the largest and most pointless post of all."
Type 1: "We're better than Casual and Advanced players because we have fun without worrying about how big our posts are or whether or not everyone is following all the rules of grammar and such."
Type 2: "Casual and Advanced players are awful because they're a bunch of whiny nitpickers who demand perfect writing skills for something that's supposed to be for fun."
Notice how the paired statements say essentially the same thing in a different way. That's exactly what I mean about there being two different ways of being elitist. Notice too how it's all talking about the people rather than the actual requirements for the section. That's another part of elitism: it's about valuing people more or less than others because of certain attributes.
On a related note, having requirements for a roleplay is not, in fact, elitist. Having standards is not elitism, it's a set of preferences. Looking down upon others is what makes someone elitist. Saying "if you want to join this RP you have to commit to posting X paragraphs minimum with Y standards of grammar at least once every Z units of time" is just putting down the baseline for what you want out of the RP you're running. Saying "you're bad, gtfo of Advanced if you can't meet X, Y, and Z requirements" would make you elitist though. For this reason I say the term High Casual is not elitist, it's simply a quick and easy shorthand for a particular set of requirements the GM wants to see in their RP. It does sound as if it's putting these players above the rest of the Casual players, but that's a problem that's endemic of the site itself; Advanced does indeed sound as if it's better than Casual, and Free being below Casual gives the connotation of less value, High Casual is just adding another step in the ladder rather than creating the problem. Anyway, I find it a little silly because High Casual requirements are without fail at or above the minimum standards for the Advanced section, thus they should just be in that section rather than in Casual, but whatever. People have their reasons for using it (different user base, more exposure, etc.), so they can do so all they like for all I care. The same sort of things about elitism apply to the High Casual folks just as it does to everything else: wanting and following certain guidelines is fine and not elitist, devaluing others because they don't meet those criteria is bad and elitist.
I'll admit to being a little elitist. Some with other definitions would call me very elitist, but this is not so by my own definition. I'm something of a Grammar Nazi and have fairly high standards for RPs. I'm not the kind of person who'll only join Advanced games (though I prefer them), nor will I only join games where everyone is assured to make 10+ paragraph posts every time (though I do make very large posts myself rather frequently), but I'm just shy of that really. If an interest check/OOC OP has a bunch of typos or is generally poorly written I won't join no matter how interesting the plot sounded. If most of the people expressing interest in the game look to be poor writers I won't join it. I've never gone into the Free section for anything other than people linking me to particularly bad threads they want to share for the amusement and one silly little RP I did with people from an Advanced game I was in that was essentially a mockery of Free RP stereotypes. That sort of stuff is why many people have called me elitist in the past. However, I don't generally look down upon people who don't meet my standards. I may not want to play with them but that's not me thinking or saying that I'm better than them, that's me having standards for what I want from roleplaying partners. My bit of true elitism comes to native English speakers whose posts consist of one line full of typos and broken grammar; people to whom English is a second language have a good excuse for not being skilled at the language, unlike those who have used it all their life. Those people I do indeed think less of, thus I'm a little elitist.
So that's my take on it. I could go into the who elitist witch hunt thing or how there are harmful and benign elitists or a number of other related subjects, but I'm off to go do the Thanksgiving thing shortly. This post works well enough as is, I suppose.