The following blurbs (from the road-map) is what has me the most excited (taken from pages 13-14):

Following the completion of the tutorial, user life-cycle management will be employed,
classifying users into various segments based on post-tutorial task completions and
achievements unlocked, and email and mobile notifications will be presented to various
segments to encourage and reward task completion based upon our own, data-driven
research into the relative retention value of the target segment.

Steemit may even promise and provide Steem Power bonuses to various segments to incentivize pursuit of certain achievements (perhaps larger ones for particularly difficult achievements, or a series of small bonuses along a chain of demonstrated retention-improving tasks).

This could be HUGE for user-retention.