I'll throw my "rejected from YC" story into the ring: my co-founders and I applied to the S12 batch. We were fortunate to get as far as the interview phase, only to receive a very kind rejecting email from Garry Tan that evening. His basic criticism was: "you guys aren't serious about this idea" (we hadn't yet quit our day jobs).
So we quit. We spent the summer building Meldium, reapplied for W13, and were accepted - and it was an amazing boost to our company. As a founder, you're going to get rejected a lot - use it as an opportunity to learn and grow.
Context: We sent an application in for S12 and S13.
Interesting that you mentioned "serious." Looking back I don't think I was a whole lot "serious" the last time I applied, but I think my application might have been better (see my comment below - got more video views last year, but that could have been an anomaly).
In any case, I wonder if being in school this time hurts our chances. Most of our founders are graduating, and the rest have signed on to jobs. But we are more serious about it this time around - hacking away on public transit, on campus and pulling all-nighters to get features out.
So we quit. We spent the summer building Meldium, reapplied for W13, and were accepted - and it was an amazing boost to our company. As a founder, you're going to get rejected a lot - use it as an opportunity to learn and grow.