Biggest difference for me is, that I had already been taking notes in markdown since before I heard about it. The only change for me was the welcome relief of not having to be creative any more about what headings or links should look like. Rest of the syntax I already used because that's how I've always made text files "look good".

As for your orgmode example, it's not a very good example of "just plaintext" if the metadata code in between the bits of text is larger than the content of the note. I imagine that with nice syntax highlighting it becomes easier to parse in just a glance, but then it's no longer "just plaintext" in the same way that e.g. YAML isn't.

It probably works very well, but to me it looks a lot like a plaintext encoding of a database record or something. It's a different use case. Not a simple note that is "basically plaintext". For instance I would not feel entirely comfortable editing your example without worrying I'd mess up the structure or produce a syntax error. Though maybe the format is really forgiving, I can't tell. What happens if you forget the :END: bit, for instance?