I'm gonna tackle this question into two parts since you specifically asked for Steemit. 

Probably the most important feature currently lacking on Steemit.com is the "Notification Feature". Around 90% of all users first platform is also Steemit.com and it still boggles my mind how come it lacks so behind to others features-wise. For most newbies this is highly discouraging to use because when a platform is marketed as a "social media blogging" one we expect that we can easily get connected to our followers and the people we follow and the best way to do that is through notifications.

I am just glad that other Steem platforms have already implemented the basic notification feature but truth be told it still is very lacking, the only service probably closed to perfection is GINABot which unfortunately requires you to download and have a Discord account. Tskkk

In terms of Steem blogging, I'm gonna go ahead and say that it lacks the categorizational function we often see on blogging platforms like WordPress. Steem platforms needs to implement a way to allow users to organize and categorize their posts properly. No one likes to scroll way down just to catch up on other people's blog. Another way to sort one's blog posts out and not the default "Newest created post first to oldest".