No idea about the statistics unfortunately. Just the fact most publishing platforms has visual editors nowadays.
Features that come to my mind right now (on top of basic features):
tables, sub-text, footnotes, math equations, images with upload and caption text, alerts ("this page needs this or that"), numbered and bullet point lists, code blocks with syntax highlighting, (block) quotes, table of contents, front matter (for homepage, author info, etc), and all the nostr stuff described in NIP-54, including wikilinks. The terminology was grabbed from here: https://docs.gitlab.com/user/markdown/
At least that's for start. I also believe that a nostr flavour markdown spec would be helpful for wide adoption and tool interoperability.
I created an article for it in tribewiki for quick listing the features: https://tribewiki.org/Markdown-fmt