Of course modern investors (or a least the ones at VC) probably aren't qualified to judge applicants on merit.
1) folks who follow the directions on the application, 2) have the technical ability to build, 3) demonstrate high achievement in school/career, 4) show commitment/persistence, 5) have an idea that makes some sense, 6) are able to clearly communicate the idea, 7) have good team dynamics, 8) have some plausible strategy for growth
AND are willing to sell 7% of their startup for 125K
then the resulting pool of startups is probably smaller than you would think.
but YC has always been pretty vocal that they can identify the right people. And their results seem to bear that out at least to some degree (or maybe it's all just scale, and giving opportunities to enough people means you'll give opportunities to the right people often enough to balance out the cost of the wrong people)
But identifying the "right" people is easy?