Ask HN: Build-Measure-Learn. What do you measure? What have you learned?

I've read the lean startup. I've seen people post stories about how they've used it to grow their companies. I still feel like there aren't enough specifics out there. Build-Measure-Learn is still this vague concept. So how do you apply it in your business?

What do you measure in your product? Why that? What's the last thing you learned from your measurements? How did it change what you built?