You have to optimize for what they want and considering how transparent the application process is, it's actually quite easy. PG's essays are step-by-step guides on how to "tick" all the right boxes. They even give you the list of ideas they wish to fund. How nice of them... No wonder YC has announced that quality of applications is increasing. It's not increasing because startups are getting better. It's increasing because more startups are optimizing.
Remember Nikki Durkin from 99dresses? She didn't just apply. She spent huge amount of time researching partners, their history, what they liked, what they wanted to hear... and she has delivered just that like a rockstar. How many promising founders are rejected because they just didn't bother to optimize like Durkin?
To me, if you get rejected, it just means you didn't put enough time into the application. If your goal is to get into YC, then keep trying, you will figure it out.
But in the end, if you are real entrepreneur, figure out what you are doing with your business. Don't optimize for YC, just do what you've got to do and if YC wants to join you for a ride, then fine. If not, too bad for them.