Is your app social? Can users share their folders with other users? How often are you notifying your users via email of new features, reminders, etc?
We email users every time there is a large update to the site with a fairly high open rate. However, not many of them return to give us another go.
A great v2.0 feature would be to use NLP and automatically tag and filter the user's tweets. It would be hard feature to pull off but it would take your product to the next level.