Few things that I can come up with on top of my head as I'm using Steemit.
Android/IPhone app: As I'm using steem more and more, I want to be able to access it in more ways. I want to be able to checkout streemit from my phone when I'm on the go.
Notifications: I'm not seeing any notifications when people reply/upvote my thread. I think that's something we need asap.
I think it is user profiles, private messages and bookmarks.
See: https://steemit.com/steemit/@aleksei-filonov/3-must-do-things-for-steemit