Nice list. It's always good to be able to quantify your progress - which can definitely help get young companies through rough patches where it might seem there's no growth happening.

I would argue it's the most important - if you don't know whether or not people are sticking around, the total number of overall new users is somewhat meaningless

Agreed - it just takes a small amount of time now to start tracking, so in the future you'll have a clear idea of where you began and how far you've come.