Well, I'd have to disagree. The only difference between a YC startup and a non-YC startup is (1) access to advising resources, and (2) a bit of pre-filtering due to the selection process. Of course, there is also the $15k of funding-- but for many entrepreneurs, getting 10-20k of seed money is really not too hard (you could
probably drum up that much "dumb" money from family and friends-- after all, how many jobless or underpaid teenagers do you know who were able to persuade their folks to buy them cars?).
For your statement to be true, it seems that either the advice dispensed by YC advisors is sufficient to transform a bad idea into a good idea or alternatively, that the YC selection committee only selects ideas destined for success. While these points do hold /some/ water (good advice never hurts, and pg et. al. clearly know a thing or two about entrepreneurship), it is still important to note that YC makes mistakes from time to time and that (participated in YC) -> (good startup) does not imply that (NOT participated in YC) -> (NOT good startup). Heck, even the first implication is a fallacy.
Regardless, at the end of the day, you're asking me to assume that founders who either don't make the cut for YC or whom didn't apply in the first place are either inferior entrepreneurs or are pushing a bad idea. This is simply not true.
Entrepreneurship has been around for a LONG time and plenty of people are working their butts off for a shot at launching a successful, growing startup. For some, YC provides the perfect opportunity to actually sit down and execute an idea; for most, YC is either not an option or is simply not necessary.
In the past couple of weeks, TechCrunch has reported on YC startups with surprising frequency. Given that it is fairly difficult to get TC to profile one's startup, it seems a little suspicious that whenever a YC app launches, it gets a (typically positive) write-up. In light of this, my conclusion that YC and Arrington have some sort of friendly or financial relationship leading to a lowered-barrier-to-article for YC-funded startups is sound if not altogether true.