Some more of my thoughts on this topic, i.e. tagging and organizing in general:
First of all Metadata is essentially data about data.
Metadata is infinite - metadata itself is data, it is possible to create metadata about metadata, metadata about metadata about metadata and so on.
In digital world everything could be both data and metada though, depending on the context...
Data and metadata can change their roles... the only distinction between metadata and data is that metadata is what you already know and data is what you’re trying to find out.
From David Weinberger:
There used to be a difference between data and metadata. Data was the suitcase and metadata was the name tag on it. Data was the folder and metadata was its label. Data was the contents of the book and metadata was the Dewey Decimal number on its spine. But, in the Third Age of Order (see the previous issue), everything is becoming metadata...
Hi May, I'm coming late to this party, and I haven't carefully read everything after the above quote.
You pose an interesting question.
However, I have to strongly disagree with your assertions about metadata.

It is a simple matter of definition. I subscribe to your first definition: "
Metadata is essentially data about data"
Therefore, by definition, everything is NOT metadata.
Simple example: The author and date of an article, book, etc do NOT change the contents of the article. If you change the author or date, the contents are still just as valid or invalid. The author and date may influence your evaluation of the contents, but the contents stand alone. It is like taking a blind testing of wine. The wine tastes the same regardless of its label.
Next, metadata is NOT infinite. We choose what metadata we think is important.
In the end, that is what really makes metadata important and useful.
We reduce thousands/millions of words down to a few select fields/values that are useful to us.
When you take a digital picture, the camera encodes selected metadata.
When you process the picture later, that is all the metadata you have.
Now you may analyze the picture and determine some technical attributes, but that is NOT the same as pre-defined, provided, metadata.
Having selected, limited metadata is both the benefit and limitation of metadata.
So, when a system is designed, content and metadata are clearly separated.
Don't confuse the ability to do a full text search with metadata.
The metadata that Evernote provides is very, very limited, IMO.
For example, many of us would very much like Evernote to add a metadata field: Due Date.
Metadata is only useful to us in retrieving data if we can remember (or be prompted) the metadata values when making the search.This is why, IMO, that use of Tags is far superior to use of keywords in the Title.
I can easily get/see my list of Tags to select the tag I want to search on.
I don't know how one does this with Title keywords. That doesn't work with my memory.