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.