I thought sales was just sales and if you could sell one thing, you could sell anything. What I didn't realize is that selling B2B to a small business and selling B2B to an enterprise is as different as a skill set as writing compilers and building web scale applications. It takes years to get really good at each one of those things.
I've also learned that as a founder/CEO, most of my time is spent selling. I was a software engineer for 10 years and now I pretty much spend all day every day selling. Selling investors on why they should buy 25% of my company, selling potential hires on why we are the next big thing, selling the board on our new plan, selling customers on our product and selling employees on the company vision.