As far as I can tell the founders' personalities are about the same as in the Valley generally. If there's a difference imparted by YC, it's that they're more confident. But confidence usually makes people more willing to take risks, not less.
Not always. I know many people whose "confidence" was predicated on success -- such as being accepted to Harvard, Y Combinator, medical school, whatever. As a result they felt compelled to be successful -- and ended up doing things like becoming a lawyer or selling their company for $1 million because they were too afraid of being "failures."
For the startups to be less of a commodity, perhaps you could look for some sort of unique perspective on the world that founders should have. It seems Google founders had a lot of that. This may not help with the returns though; maybe more money are there to be made from commodity startups than truly disruptive ones.