I've seen this a bunch of times, but I like that focus on Activation and Retention for new startups.
Ash Maurya's Lean Startup book is interesting in that it tries to quantify when you can be ready to launch a new product: when you have 40% user retention. But personally that seems high to me if your early users are full of random joes who've signed up to some email capture form that promises rainbows but they don't really know what they signed up for.