In an interview with CapLinked.com (which Thiel is an investor in) Peter Thiel stated "My general analysis of my PayPal experience is that if I knew then everything I know now about the payments space, I would never have started the company. It would be too intimidating. There were a lot of extremely difficult challenges we didn’t expect in advance. In that crucible, I saw how truly transformative breakthrough technologies can be. And then there’s the macro-reality of economic growth, which holds that things only get better the more a society innovates."
The interview also talks about his investment in Facebook and is worth a read.