Don't use Rockstar or Ninja! OK in all seriousness, For me at least (before I became a freelancer) talking about the work environment, the people and the app would attract me more than the gory details of what you expect. Most everyone that writes apps, knows what is required of them. I would keep it simple, what you want them to do and what level in their career you want them to be at.
As well, I would include what you are willing to pay if it is below market (not that I am saying that you are paying below market) that way expectations are set before time is wasted on either side.
Yep, that would do the trick. It's like porn for some of us. In the end people want to work with other cool people. Hence the Ninja and Rockstar post, when you are not cool, the best way to try to fake it is with some Ninja or Rockstar slang. But in the end posting some shots of the UI and talking about the people / work environment will reflect a more positive light. Then trying to "cool up" your job posting.
As well, I would include what you are willing to pay if it is below market (not that I am saying that you are paying below market) that way expectations are set before time is wasted on either side.