As somebody who only makes a pretty basic use of org-mode I disagree.

Without having to read anything you just have to remember to use * to mark sections and * * for subsections and so on. Then you use tab to expand/collapse the sections. That's rather intuitive, not more complicated than a plain text file and already you have better highlighting and additional functionality.

Then sometimes I wonder "hey, I'd like to have a link to this URL in this file" or "I'd like to export this to HTML or PDF so that I can send it to my friend who doesn't use org-mode without losing the formatting". And then you look in the manual and you (generally) find what you're looking for.

Sure if you want to be an org-mode wizard you'll have to learn quite a bit but I really don't see "the complexity penalty vs. using a plain text file". The complexity is only here if you want it.

Edit: also the array auto-formatting is a godsend. Doing it by hand in text files is tedious.

It depends what you mean by "good use", I suppose; I started out with just the very basic outliner and checklist features, and expanded from there. It didn't take much more than knowing those features existed, and what they looked like, to start using them. You do need to refer heavily to documentation when getting started with more complex features, but that seems like a fair tradeoff to me.

I'm also not sure what you mean by "complexity penalty vs. using a plain text file", since Org files are plain text files, with all the magic implemented in the UI layer - I regularly edit Org files on my phone with Editorial, and while the UI is obviously rudimentary by comparison, such editing is not actually difficult to do. Will you elaborate on what you mean here?