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The main thing they appear to look for is viable business, future vision, then team. I highly recommend applying."
It seems to me — based on YC's resources I've read — the passion, personality, and relationships of the company founders come first— including how willing they are to be flexible with their idea.
Jessica Livingston on what they look for:
https://www.youtube.com/watch?v=SPd5vgXJ-R4
They also often say how ideas are a dime a dozen, mentioning time and again that most businesses who join YC end up changing their idea completely. They even went as far as to accept founders without an idea at all one year:
http://paulgraham.com/notnot.html
"In a sense, it's not a problem if you don't have a good idea, because most startups change their idea anyway. In the average Y Combinator startup, I'd guess 70% of the idea is new at the end of the first three months. Sometimes it's 100%. In fact, we're so sure the founders are more important than the initial idea that we're going to try something new this funding cycle. We're going to let people apply with no idea at all. If you want, you can answer the question on the application form that asks what you're going to do with "We have no idea." If you seem really good we'll accept you anyway. We're confident we can sit down with you and cook up some promising project."
more related articles from the Paul Graham:
http://paulgraham.com/start.html
http://paulgraham.com/bronze.html
http://paulgraham.com/ideas.html
http://paulgraham.com/ds.html
On the other hand, Sam Altman discusses, in the Stanford YC Startup School videos, how the overall idea certainly does matter and that thinking "the idea doesn't matter" has become a myth in silicon valley. He says the most successful companies have been an idea first, and a startup second.
https://www.youtube.com/watch?v=ZoqgAy3h4OM#t=22m52s
I think the answer is somewhere in the middle, leaning in one direction depending on the founders' mindsets and knowledge. I wish they would be a bit more clear about this since both opposing arguments are being advised through their resources.
Either way, I look forward to start applying to batches when I feel ready. Perhaps I'll apply without being ready to help gain some clarity as you suggested.