It would be fabulous to be able to share a notebook with specific users in a collaborative fashion. Being able to make a notebook public is great, but sometimes you want to only show it to a few people, and if you could opt to allow them to add data of their own that would be even better.
Subnotebooks would be great! I see their functionality as being different than tags. If you're relying on tags to replicate this functionality, then you'll end up with an unnecessarily long list of tags, and I'd rather be able to use tags to overlay another form of organization over my collected data. And speaking of tags, they're not that useful if I can't select more than one at a time, e.g. Family and Vacation and Tora Bora.