Thank you for making this. I really like it.
A long time ago in a startup far far away a friend and I built something like this. Ours was nowhere near as elegant. We would look for CamelCaseWords to kick off the autocomplete but your use of @ is much better.
Delightful moments: pasting an image url embeds the image. Consider letting me backspace the image to get back to the url.
The multi-user edit seemed very smooth. Better than collabedit to this user.
The task list is nice. I'd love to have a title to the task list and be able to query for and surface all task lists across pages.
Please consider supporting #hashtags if only so you can show a tag cloud. The built-in search is great.
I couldn't figure out how to link to a person.
Finally, for users who login with their own email address, please consider letting them in right away. At most have the confirmation step be something they can complete later. Eliminate friction at every turn.
All in all a really nice product.