Actually, I think this is the most important part of the process. When you're small you have the luxury of making mistakes and no one cares. When you wield some power/prestige and your actions affect other peoples' lives, people start coming to you with grievances. The worst part is that often those grievances are "deserved" --- I guarantee that some startup w/o an interview request will make it big, and one that gets accepted will slink off and die. That doesn't mean that YC is failing, just that the problem is really hard.
The problem itself isn't very hard, at least not in the mathematical sense of the word. It's just that it's a human process, and is thus prone to the failings of human beings, including the various biases that human beings are known to have.
