I'm not going into the philosophical reasons that I want sub-notebooks, but just know that for what I'm doing and for how my brain works I would really appreciate it if they were added. Keep all the good tagging stuff too, but just give me a concrete place to stash stuff quickly.
I decided to post here because I was actually trying to use Evernote today - trying to set up my note structure and become a consistent user (and likely future suscriber) - but became frustrated with my growing list of tags. I kept wishing there was a way to make a sub-notebook. I KNEW there must be a way because it seemed like such a basic UI feature. I couldn't find it anywhere and consulted google only to find out that there are no sub-notebooks.
I think of those multi-section divided notebooks that I used in high school. I keep each subject in a section of the notebook. Each day I easily flip open to the correct section and write my notes in the section that belongs to that class. When it comes time for a test everything is there, in order, and easy to find. Now, say I have to write a paper for another class that incorporates things I've learned in several classes. I tear out all the pages I need from various classes that are about the same subject and put them in a 3 ring binder to write my paper. This is how I would like to use sub-folders and tags.
I know that with work I could do that with tags alone, but I think that this is how it works in so many other places and just seems natural. Maybe my mind is too feeble to understand the benefits of a "tags only" system, but for me sub-categories just make sense and would be ideal working in conjunction with your powerful tag system. Why not give your users more power by implementing both?
So, to sum up... +1