The further I get into the industry (and that's still not incredibly far, yet), the more I hear people talk about the importance of source material. If you're unsure what I mean, 'source material' here describes existing material, which your script is based on. It can include anything from books or graphic novels, to other movies or even paintings. Basically, it's something that studio heads and producers can look at as already having found an audience (even if it's a very small one), which makes them more comfortable with backing your big idea.
A prime example is Larry McMurtry's Lonesome Dove (and my favorite western). He wrote an epic script about rangers who drive a herd of cattle from Texas to Montana. No one wanted it. He then wrote an epic - 900+ - page book based on it. It was an in-demand property. He quickly tweaked his existing script and sold it as a four-part miniseries. Go Larry.
What this all means, more or less, is that your awesome superhero movie idea, while indeed quite awesome, is less likely to get greenlit than a movie about a third tier superhero that a dozen people read about each month is. Why? Because the product is already in place. More than that, there are already visuals that the writer (or director, or producer, or whoever) can produce in a pitch meeting. You have an idea - and even a script - which is great. What you don't have is a comic book you can point to and say, "look, here is is, hitting something!" It sucks, but I'm finding it's true.
In the past few months, as I work out future projects I want to develop, I find myself butting up against this 'source material' wall. Superhero stuff - unless a rarity like The Incredibles or Hancock - is basically not gonna happen without a comic book it's based on. Giant sci-fi epics (unless you're James Cameron) are in the same place. For emerging and unproduced writers now, the waters of the market are cold and rough, and a simple, one-sentence pitch of an idea are (perhaps unfortunately) some of the safest boats. Heat worked, so how can you do Heat with a unique spin? That's what you'll be asked.
Of course, I don't want to limit anyone. I was talking with Onyx about this last week, and he reminded me of something important, worth keeping in mind. Sure, it's tough to launch your own The Matrix without source material, but a couple big sales can make all the difference. A tried and true writer can get a lot more experimental than a newbie. So, for the time being, I know that some of my ideas are going on the back burner. I'll keep working on them intermittently, with the hope of one day being that tried and true writer. Then, look out. Something awesome will happen.