As the original poster revisiting this thread, I think a lot of my frustration comes from the moving target aspect of element names.
That is, just as there is an API for plugin development, I personally believe a well-known and consistent theme needs to be developed that uses a thought out and extensive set of CSS elements which are named sanely and have descriptive comments.
What I see, instead, are three options: 1) Use someone else's theme as-is 2) Use someone else's theme as a starting point (VERY common) 3) Roll your entire own theme from scratch.
Option 2 would be well-served by theme developers, or at least ONE theme developer, treating their theme as source code that needs to be well-understood by thousands of people instead of treating it as for their eyes only with an "I can't be bothered to spend another 1 hour on this to make it 90% more usable by people". That theme developer would have to know WP guts very well and try to be as comprehensive as possible. I routinely find many half-complete stylesheets that do not even stub in all of the WP-provided elements available for tweaking.
I understand VERY well that this is all volunteer work and contributions from enthusiasts. If I were a CSS guru who already had his fingers flipping through the WP code, I'd have done this already and offered it. Unfortunately, I am far far from it. I'm just a guy who sees room for improvement, and that's all I am proposing.