True - I hope I didn't come across as bitter, overall this was an awesome experience and the people at YC are amazing. I actually do think that they can probably tell if a startup is a good one or not in a matter of minutes.
What they are looking for is probably a subset of good startups, ie. your startup can be good but it's not what they are looking for.
To some extent it must be a statistics game for them. The investment they make flying founders out must have a good return for them in terms of potential investments. Since they have been doing it for a while now the numbers must be close to optimal.
Yes, you sounded very bitter with the process and contested strongly how YC decide if a startup is a good one or not.