Tree structure for notebooks, please. This really is a significant feature. The metaphor of a notebook is a necessary metaphor: One place to hold everything that the user wants to "bind" into one place, one conceptual bin, ordered chronologically. Tags are a different concept: cross-references between things that the user does not necessarily want to put into one bin--tags label things that may not have chronological or immediate conceptual unity for the user, but which have some referential relationship. The restrictions of the UI make a flat notebook hierarchy unwieldy. At about 20 notebooks, things start to get messy. Subnotebooks of arbitrary depth let the user use notebooks to bind things together as needed, in a clean, easily manipulated way. Tags let the user cut across the tree notebook structure to mark inter-relationships and retrieval possibilities that a notebook tree structure alone does not allow. These are two different approaches to data, both with their strengths and weakness. Both are useful. Right now, you are severely diminishing the usefulness of the notebook approach by not allowing a notebook hierarchy. Even if you are not persuaded of this way of viewing tags vs. notebooks, please still consider putting notebooks in a tree structure just because users are asking for it. I'm also in the "give me tree structure for notebooks, and I'll go premium" camp.