This is actually a fairly good topic for discussion, so something tells me that you already have some idea of how to attract people into discussion of your vision--hence, you can write a decent interest check.
Myself, I hate people who can't think of a title more hooking than "partner wanted" or "ideas inside". If you have 10-15 things you want to play, that gives me the impression that you don't have ideas for any of them. (Okay, I can think of ONE idea you probably have in mind for them...) The title of your thread is the first chance you have to draw people in! Take advantage of this!
Next, I want to see good spelling and grammar. While I personally know a few otherwise intelligent people that don't believe in the necessity for this sort of thing as long as they manage to communicate their ideas, I maintain relationships with them because I got to know them before I got to know about their writing. Over the internet, I don't normally get this opportunity. There exists software to correct the majority of mistakes you'll make in your writing career, and if you don't want to go to the effort of using it to make yourself look like a better writer, that tells me a lot about the amount of effort you're willing to put into your roleplaying.
Finally, I want an actual, concrete plot with a beginning, middle and end, not just "three girls fall for the same guy, what will happen?!" This sounds like an easy thing to do, but a brief overview of existing interest checks will most likely put the lie to that. I want to know what my character is supposed to do from the first post--it must be noted, however, that this is very easily taken too far. I don't want a GM to tell me who my character is supposed to fall for and why. Not that there isn't precedent for this sort of behavior--screenwriters and tabletop GMs that distribute pre-made character sheets do much the same thing, and in the latter case it's a great way to introduce a player to an unfamiliar system. I just think it has no place in this medium.
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