"Build your product to solve a customer’s problem." was absolutely revolutionary for me, I'd never even considered doing that, as it's completely counter-intuitive.

But when you think about it for a few hours, it kinda makes sense. Then an alarm goes off in your head and you're like "wait a minute, if I make something that solves a customers need..... they might use it". That was an absolute turning point for me after I read this very article 5 minutes ago.