Silicon Valley loves its engineer-founders. They are members of the region's highest caste, the entrepreneurs trusted to turn bits and bytes into the next hit digital products, and the people venture capitalists most like to back.

Stewart Butterfield, co-founder and chief executive of the workplace chat app Slack, is not one of them. He stands out as a philosophy major in a start-up world full of software engineers, a non-techie who has made it to the top of the tech heap.

Slack's listing on the New York Stock Exchange this week has cemented his reputation as one of the Valley's