Other than tags/search, one thing that is very important to me is "first-class" math support, where I can enter LaTeX math notation that renders inline, but which behaves like text whenever the cursor enters the math block. In general, I always appreciate when editors locally "act like" a plain text file near the cursor, but abstract away all the markup wherever I'm not directly editing. Two editors that get this right are Typora (inline-level editing) and Jupyter notebooks (block-level editing).
I agree that ProseMirror has a steep learning curve, and there aren't currently enough open-source extensions available. Hopefully, with time, enough standard components will emerge. In the future, I hope to see a ProseMirror-backed drop-in Markdown editor become the default for browser-based editing.
ProseMirror doesn't currently have standard support for wysiwyg-math, but I've recently been working to change that [1]:
[1]: proof of concept: https://codesandbox.io/s/quizzical-austin-m7n3t?file=/src/in...