So, just before you get to far into this: this is going to sound a little whiny. But I promise you, its not. (At least I'll try not to be.)
I've played quite a few Rps on different sites, of varying qualitys, and I've enjoyed a lot of them, some ranging from me and a few other people, to one I'm involved in right now that has 15 active people going at once. (I'm new here, so I can't judge if that's unusual, but I think that's a lot of people) And so, I thinks to myself 'hey, why don't I make an Rp?' So I do. And I have a lackluster response. I get a few people joining, and it goes for a week, and then everyone loses interest. And so I thinks to myself, 'hey, maybe people aren't interested in that game idea, so let's do a new one' and I do. And I get even less of a response. This has happened a few times now, and it frustrates me, since I think I have some genuinely good ideas and universes that would be fun to Roleplay through. But to even get started, I have to have people signed up to play. Which is where I've failed each time.
So how do I get people interested in signing up for one of my games? What's the best way to throw an rp pitch to get the phat l00t Roleplaying ball rolling?

In my opinion you need three solid things:

-A good setting and plot.

-An aim.

-Ability as a GM.

All three of these things are conducive to each other and I'll break down what I mean. What is your setting? Is it something original? If not is it at least something not being done much at the moment? There's no truly original ideas any more, but is your is your RP just another 'x' type of RP or does it have a hook. The forums can at times be saturated with samey RPs.
What are you trying to do? What's the aim; the core philosophy of the RP? Is plot central or is player interaction or perhaps combat/a sense of winning? Has this core ideal been conveyed to potential players?
Probably most important though is how good are you at actually selling the RP, getting players motivated and keeping things organised, essentially, your own abilities as a GM.

Perhaps share the specific roleplay and more particular feedback can be provided.

@Darkmatter
Perhaps my nihilism is a bit misplaced, I just rechecked the interest thread, and a few people have noticed it.
I'd still like to learn how to be much better at all this, I want to know why some stuff works and what doesn't.
http://www.roleplayerguild.com/topics/84007-an-expedition-beyond-the-world-a-low-fantasy-exploration-game/ooc#post-2873057
Tell me, is my opening thing to brief? I've seen a lot of other interest checks with full, page long IC hooks and whatnot, and I only have a few paragraphs.

@Sypherkhode822

I think I found your problem .

The cyberpunk RP you joined, which I am in the process of applying to, had a ridiculously quick start. Stupidly so.
2 IC posts, 9 characters and 100 OOC posts in just over a day? That is not normal, that is exceptional and gave you an incorrect view of the usual speed of things.

Four people responded with interest to your RP within 24 hours, and that was with a paragraph of a concept. Compare your interest check to the opening/setting post of the cyberpunk RP.

Get a decent opening/setting post up as an actual RP thread, link it to the interest check and you're sure to get a good number. Four people is plenty! That's five including you which I would say is definitely the most normal number to see. You did nothing wrong you just had high hopes. Four players interested in twenty fours is great! Flesh out the idea, and I'm sure you'll have no problem getting it going.

@Sypherkhode822
Four players interested in twenty fours is great! Flesh out the idea, and I'm sure you'll have no problem getting it going.

Oh man, thank you so much. I have no baseline for how this works on this site, so you telling me how it works is a huge relief. :)

No worries man. Asking questions is how you get ahead.

I do say this a lot, but when it comes to the OOC setup, make sure that you use a good layout with titles, lists and formatting. It'll help convey your message to your audience a lot better as it's more like documentation for your RP.

@Darkmatter did mention some good points with setting and plot, an aim, and the ability as a GM.

Your setting needs to be well thought out to the point where you can say definitively yes or no to most questions (eg, Will there be magic? Can I use this weapon? I want to go down this route? etc). By knowing what it is that you want and sticking to that, you can prevent any complications occurring later on with bias, overpowered characters, and Mary Sues (Characters that fit TOO perfectly into the world).

It will also be 'your RP' so as a GM you need to know when to say no and when to say yes. Showing that you have a clear vision will draw people in as they will feel comfortable that the RP will work, and not become some hodgepodge of ideas that 'may' work.

Finally, the Aim. Probably one of the most important factors that seems to fall flat on many people. As a GM you are creating the world, but giving a motive for the players in that world can be a bit difficult sometimes. I have the best example in one RP where for a few post my character didn't have anything to do as she was waiting for the GM to supply a new Aim after I completed my last. Instead of giving me a goal she wanted to supply exposition for the overarching story and thus my character had to wait for a few post for no particular reason. To be truthful, no one really knew what the goal was going to be as she only created the world and kept all the story cards close to her chest.

There is a fine line that you have to tread when it comes to GMing and that is between Roleplaying, and Story Telling. If you're too heavy into the story telling your players will notice it and feel like pawns rather than players. Therefore, if you mention in the OOC that there is a distinct objective of some sort, it will help portray your RP as something that has choice and opportunity. I won't go too much more into this as there is a heap more that I wouldn't cover for your needs, but by keeping this in mind while you create the RP, you will begin to understand where the parts are that you need to focus on.

Best of luck and let us know how you go.

Problems, put bluntly:

1) You posted in the Advanced Section. Advanced gets the lowest traffic. You minimised your pool before you started.
2) Honestly, it doesn't look like an Advanced interest check (see #3), so you might well be passed over by people who might think the RP should be in a different section.
3) It really isn't the best interest check in the world. Standards for Advanced are typically higher (ie: potential players expect to see something more impressive), and the level of detail is quite thin. It's set in an original world with barely one paragraph of geographic detail but almost nothing on history, culture, etc. I don't do fantasy, but if I did, I wouldn't have the faintest idea where to begin. The Codex is a great tool, but it's hidden on the fourth post.
4) It also isn't the most attractive interest check in the world. The title is clear but a little uninspiring, and there's no formatting at all. Formatting interest checks means you can lay out information clearly and attractively so people can easily get the information they need.

Solutions, for next time:

1) Try Casual. If you like big games and want lots of interest, Casual really is your place.
2) If you stay in Advanced, watch your grammar and format your post. Think less 'fun thing to do' and more 'business proposal' until you're confident you know what you're doing.
3) Give a concise description of world-information, plot-information, and thematic information along with a tl;dr that doesn't include references to stuff some people might not get, and give a clear link to any external sources with a 'for more information' message all in the first post. You kind of have all the bits and pieces, but you need to polish them, which brings us to #4
4) Hit it with the Pretty-Stick. This is a great resource. Some people just won't read an Interest Check if it's badly-organised and some others (like myself) will find the information very hard to take in. Leave carriage returns between sections. Have headings. If your Interest Check is badly-organised, by the way, it gives the impression that you're a disorganised GM, also known as the worst thing imaginable to the role-playing population.

Luck. You need the right players to be available and looking at the exact time that you post your interest check. If they're not interested in the idea, no amount of fancy formatting, pictures, or flavor text will save you.

Thank you guys for your feedback, this has really been helping me get a sense of how this forum works.
I do think I'll be moving this down into casual, as I've seen that advanced can be very demanding, and that's not my intention. (Though I do want higher-quality.)