Simply put, I would find it very useful if Evernote could allow for a directory structure for organising notebooks and sub notebooks similar to how we can use folders and sub folders on Windows.
So for instance, if I want to organise notes for writing a novel, I could call the main notebook 'Novel', and then have sub notebooks called 'characters', 'theme', etc. Within 'Character' I could then have sub-notebooks like 'David', 'Sarah', etc containing notes about those characters.
Trying to use tags seems unnecessarily complicated when a simple (and visual) structure would be so simple and effective to use. Perhaps Evernote is simply not set up in such a way to allow this, in which case fine, but personally if if this was possible, it would really help improve an already great product.