I just had the opportunity to tour the Valley a few weeks ago and talk to people. The big totally-did-not-expect-that thing that kept coming up in conversations: the relationship between co-founders is often intensely stressful and frequently a source of business failure. I was expecting that people occasionally had different ideas on directions for the business. Some of the stories put me in mind of tabloid coverage of Hollywood divorces.

Not sure if that is a funded-startup thing exclusively, as I know at least one bootstrapped startup with co-founder drama, but adding investors to the mixed seem to bring a lot of things to a boil.