No Management

That’s why Valve is flat. It’s our shorthand way of saying that we don’t have any management, and nobody “reports to” anybody else. We do have a founder/president, but even he isn’t your manager. This company is yours to steer—toward opportunities and away from risks. You have the power to green-light projects. You have the power to ship products. 

Valve Corporation's Employee Handbook, posted in an online Forum

I've heard similar stories about companies without formal management, and they all seemed to flourish with typical human resources KPIs like unwanted attrition and sick days being at a minimum. I'd love to learn more about what makes this work.