Marketing. A developer with marketing skills can build products and achieve revenues far in excess of their skill set.
I can't speak highly enough about http://tractionbook.com/ which I recently came into - it has pragmatic advice on marketing approaches and systems justifying with real data how and why to do what. It's co-written by the DuckDuckGo founder, and as an engineer who is thoroughly tired of marketing "hackers" who justify expensive campaigns with hand-wavy nonsense, this book changed the way I view marketing in general.
How do you feel?