Direct messaging. Having off topic conversations in the reply threads seems a little awkward.
Combined with the circle functionality from Google plus would be great for group conversations.
Also, games. Games are an integral part of humanity. Nobody ever stops playing games entirely. We use them to establish dominance, settle arguments, learn new things, and distract us from everyday troubles. Steem NEEDS games.
Maybe a marketplace, a place people can post things to sell for Steem. Buyer beware of course.