Pretty cool/interesting. For the most part, I've stuck with marked and try to stick as closely to gfm as reasonable. I have on occasion added additional emoji support, but that's all I've found that I needed.
Recently added a layer to present a medium-style editor over the top, and that has been a huge set of frustrations in order to filter for nefarious content (xss).
Part of the exercise is to practice writing clearly. This is a very important skill in a number of fields, in particular medicine. When legal, medical or engineering notes are hard to read, especially potentially a while after written there is the opportunity for bad things to happen or the inability to find context later on.
While I appreciate the effort and think the project is cool, I find that the purpose of side stepping part of the exercise/assignment isn't such a great idea.