How can "having more depth in hierarchy" be more restictive than having 1?
I understand your point and it works well for people who have so much information that they have to use search functionality all the time. They really need to categorize the data (tagging), that data can be anywhere, because its location just doesn't matter. These people wouldn't be harassed by even a totally flat note structure.
However project managers like me (and around me) are cursed with some data organizing habit. So I keep my projects in folders (named by the project's name), and when I want to work with something I can usually pinpont it, I don't need to search. This makes tags completely useless to me.
Then I tried to think outside the box, as you suggested , because the file-system example is wrong if we look at the fact that file-systems don't support tagging functionality at all and that's why we are used to use folders. But I immediately missed features which would ease the pain on tags:
Introducing tags in the note creation process very early: There are a lot of people arguing about the missing stacked notebook feature, and the answer is usually 'use tags'. So these people need to be taught to use tags. But - for example - if I select a tag (or more tags), the right-click menu does not contain a "Create note with tags" entry. I don't know how much of the documents are automatically imported into Evernote instead of creating it in Evernote, but I usually do the second (and not scan, send a document into Evernote, or whatever).
The illusion of usefulness of nested tags: adding the deepest entry of nested tags won't add its parents. Which makes tag nesting useless and confusing. A simple tag grouping would have been better to understand, although useless as well. Even if there were some "auto-adding nested tag's parents" feature, it would be still useless, as when tags are added to a note, they immediately loose their child/parent hierarchy (so why create them in the first place?).
I hope I wasn't nitpicking too hard, I might have done lot of UX and UI projects this year