Live View (a WYSIWYM Editor) - you type markdown, but see it rendered, comes to mind. Also some interesting extensions/add-ons centered around knowledge management are arising.
Unfortunately, Obsidian itself is not open-source though; but worth checking out.
Sync across devices is an initial benefit to Obsidian that I can think of. Being able to write notes using your phone or tablet?
You can e.g. sync with Syncthing and write notes (on Android) with QuickEdit. Or use Dropbox and some Markdown editor with Dropbox support?