The number one feature request by far at this point would be collaborative features in the notebook. Not really because I actually want to collaboratively edit a notebook with others, but because I would like to open the same notebook in two separate browser windows (side-by-side), and edit a cell at the top of the notebook in one, and a cell at the bottom in the other, and have things get merged automatically. Or, not having to worry that I left a (remote) notebook open on my work computer when I connect to the same server from home, causing parts of the notebook to be accidentally overwritten.