There are two useful features I've found to be rare among markdown editors:

1) Inline application of markup. Discord does this for example. If you write text like this, the message preview retains the '*'s, but the text inside is italisized.

2) Persistence of text formatting when pasting from rich text. Roam Research does this. If I'm copying-and-pasting some rich text with e.g. a link in it, the link will be converted to the markdown format and then pasted.

I think to have a truly great markdown editor you need both of these things, but I've never seen an open source one with support for both. I've had a mind to find one and implement them myself, I might try that with tail.writer.