I'm probably underselling it but for me, markdown support and note organisation simply reflecting the directory structure is what sold me. I can then sync it whichever way I want; I've taken to having a "notes" folder in each project and including it in the Git repository. The notes are then browsable on GitHub too.

I've spent a lot of time trying to find my "perfect" note taking app, and have attempted to build my own a few times, and I'm very happy with Obsidian.