1. I don't see details of retention anywhere, are you just storing indefinitely at the moment?
2. No sign-up is nice, but have you considered even just using cookies for 'my past notes'?
3a. ```Code blocks``` render as `inline code` in editing mode, which looks worse than not doing any 'preview' of them in edit mode at all IMO.
3b. ```lang hints (in the manner of Github flavour) are ignored, though some syntax highlighting is done with or without them.
3bi. This seems to mean that keywords from other languages are highlighted as if a keyword in the language used.
3bii. The syntax highlighting colour scheme used is not very colour-blind friendly.
4. Two users (uh, tabs) can concurrently edit, but each others' edits are not seen until refresh; so last to stop typing (=> save) wins.