> Rather than take the approach of a startup to validate their idea they went full steam ahead assuming they knew exactly what people wanted.

This is the big one for me. YC more than anyone pushes the idea to launch early, and validate. Raising a huge amount of money is fine, all the power to you, but hiring a ton of people off the bat, building your app in isolation of real users, and pouring gas on the marketing before ever launching is just asking to go out in flames