Here is what I miss the most from some of the other social media platforms I play with:
I really like the ability to use very simple text decorators to create content which makes use of proper section, subsection, sub-subsection, etc. navigation markers, easily lets me link to other media, and even has the option of generating a table of contents.
To say that I like longform media is probably an understatement.
I don't mind a really good rich text editor; Medium has one of the best longform content editors in the social media space, in my opinion. But it would be really nice to be able to create the actual text, link to other content, be able to simply cut-and-paste that into a post, and have everything "just work."
I can get tiny, individual updates in a lot of other social media platforms. What I really want, especially on a topic-first interface, is something that makes it easy to make content which is as ideally presented.