The biggest change is your jobs goes from doing things (which makes sense) to building an incredible team that can do things (which is a more unintuitive job). In the limit, it’s always a people business.

Overcome many challenges, but per my last answer, building a team of the best people has been the most important and most challenging. That, and learning how to do sales ;)

Too many mentors. People in Silicon Valley are incredibly helpful. To name a few: Dan Levine, Mike Volpi, Nat Friedman, Adam D’Angelo, Ilya Sukhar, Jonathan Swanson, Albert Ni, Jeff Arnold, Charlie Cheever, and Drew Houston to name a few. I’m very very lucky.