Pivot bins do make a huge difference, and in spite of repeated requests to turn bags on their side and hold on to your jackets/coats, I still see an awful lot of wasted OH bin space. I've also noticed it tends to be more of an issue on flights with a higher rate of vacationers (as opposed to business travelers), particularly when "1 carry-on plus 1 personal item" is not enforced.

Of course, I've also seen the inverse: people putting their modest-sized carry-on in the OH bin then closing it when there's still lots of space remaining to give the impression that it's full.