I'd say the bot issue and maybe the new user onboarding. It's not easy to understand fast how it works.
It may be intersting if you produce onboarding steps at the first connexion. Besides, it would be cool if we can have some recommandation as Who to follow, who is trending etc...
I think you'll find more returning users and an outstanding retention rate!
Goog luck the Steemit Team! Love your product!