Knowing What Feature to Build Next

We're currently using this method on http://www.marvelapp.com to find out users who are willing to upgrade to Pro.

As we're still in the process of building Pro accounts, we wanted to get users who would most likely sign-up first.

We placed a 'notify me' button in the UI, working really well. After around 3 weeks we've over 100 users signup.

I was inspired by Buffer's MVP post a while back, great idea.