That's a great idea and really good implementation. The only thing I'd miss from Google Docs are comments that are not part of a text (in Docs these are rendered on right side of document). You could implement it by come special syntax for comments, which wouldn't render when exported and on site, and in edit mode, it would show just a little icon. GUI option to click somewhere to add comment in this place would make it even easier. It allows further capabilities, like easy inline peer review.