We need custom Achievements and an Achievement Tracker
Achievements are addicting. Everyone who has played games with similar systems knows what I'm talking about. It's very rewarding when you finally get that "Achievement X unlocked" badge.
SuperBetter does have achievements currently, but it's lacking the "goal" mindset in them. For example: I can't see what I'm working on, I just randomly get notified that I unlocked an achievement.
It is satisfying for me to see "Warrior lvl II" unlocked, but much more motivating would be able to see "5/10 foes battled for warrior lvl III". See what I'm going for? We need a meter that tells us how much effort is left before we can unlock the next level of our shiny badge.
And if we combine this system with the ability to create our own achievements, SuperBetter can have an awesome motivational tool added to it. Imagine creating an achievement "I know Clean-FU!" that requires you to complete your "Do some cleaning in your house" quest 20 times. If you complete your achievement, you get it added to your custom "My Achievements" category.
After this, all you need is a tutorial quest to teach users how to start settings smaller goals for them using the achievements.