IMO, it's a pretty simple formula.
1) Build something people want-- ideally, prove it. Have users. Show that they log in every day and would sooner gnaw off their own arm than lose your app. There are other ways to prove market demand other than loyal users. Find them. Leave no question that people desperately want what you're building.
2) Prove that you can build stuff. Have an impressive and interesting resume for getting stuff launched/shipped-- ideally, you can show that you've build/shipped stuff under great adversity. Show determination.
3) Show that it's a big opportunity. I remember there was a team talking about building a tool to manage D&D campaigns online. That's not a big opportunity. YC only makes "keep the lights on" money with the 10-20m exits. You should hopefully be playing in a space where there is a ghosts chance of a 9-figure exit. They're pretty lenient on this one, I think-- ya never know what can get big.
(IMO) The biggest risk YC is taking on is that no one will want what you're building. The second biggest risk is that you aren't capable enough or determined enough to get it built. Your adding the risk that you can't find a decent co-founder-- which doesn't help.