Does it make you win?

Yeah that's a good point.

Specially if you're a builder, you tend jump on building things as soon as something excites you. Sometimes it works, but sometimes the ideas might exciting but not that meaningful.

Good point. In the really early stages of a startup, it would seem best err on the side of focusing on maximizing the potential / value of "what makes you win" given the danger at that stage tends to be doing too many (unfocused) things. I still think "slack" time is important, but only after what "makes you win" is running out of juice or you need a different stepping type / level of stepping stone when expanding (say, when you cross $10,000 users and you've run out of friends / acquaintances to sign up). As you get bigger, it's probably easier to more formally dedicate people and time to "slack"

Great. Let us know what you think about it :)

Sounds like 'validated learning' to me, see http://theleanstartup.com/principles