Hierarchy of notebooks/stacks should be a given. Currently the only thing keeping from using Evernote over OneNote is the lack of hierarchy. I really, really would prefer to use Evernote, but no matter what I try (hours and hours of experimenting) nothing is right for me. Yes, I have tried tag hierarchy. I like my notes to be in one place, one notebook, and for tags to be something additional. That is how my brain works and is the genius behind applications like Workflowy and Todoist.
If hierarchical tags are enough for you, good for you, but everyone doesn't need to be just like you. There is no universal "right" way to take notes, what is right is what is right for you. A good app offers a variety of options to many different people who think in many different ways. I happen to be non-neurotypical and frankly the posts that are like "I can't see why you could possibly need this" frustrate me. Of course you can't, because it isn't how you think, but can you recognize and accept the fact that some people do think that way even though it isn't how you do?
And yes, I have found another app to use. OneNote. It isn't perfect but I have weighed the pros and cons of each application over a few years, and have in general experimented with many note-taking platforms (I genuinely enjoy doing so.) I have found OneNote to be the better choice when I weigh the pros and cons, but that all would change if Evernote had notebook hierarchy. So yeah that's the reason I have come on here to comment, because I really do want things to improve with Evernote.
Also I want to add notebooks don't necessarily have to be under notebooks in a hierarchy, it could actually just be stacks under stacks, with no sub-notebooks and only sub-stacks. Honestly either way would be nice if there were just true hierarchy.
Also something that makes a huge difference when it comes to notebook hierarchy vs. tag hierarchy is that with tag hierarchy you lose the ability to share things as one notebook. Since Evernote prioritizes collaboration a lot you would think this would be important to them.