I recently spent an afternoon at an exhibit of 15th century Italian art. Now there are all sorts of deep things to say but one fairly simple theory struck me and I wanted to try it out on you all:

The reason so many of the faces look so funny is that no one has figured out how to draw people straight on. It's the problem of invention of perspective in miniature. As a test for this, notice that the profiles become sophisticated much earlier than the full-on views (or the various strange kind of three-quarter saints with eyes heavenward, as the artistic community as a whole attempts to make the switch into three dimensions).