On a 640x480 screen with 72dpi resolution, a 16pt font looks quite large,
so it ought to be legible on a presentation slide, right?
It certainly looks like that way on the 720x576 screenshot here.
But 720x576 pixels on a sharp computer monitor is not your output target!
Your audience is either watching it on a washed-out projection screen,
or through a drastically scaled down Ogg/Theora stream. Is it still
readable at 320x240, with compression artifacts? You be the judge:
Mind you, this example was easy! It's white text on black background. Most slides have lower contrast than that. The half-resolution example is shown at 200% size, which many users will likely prefer to a small 100% window.
You can look at the example videos the screenshots were taken from:
720x576 original resolution Theora video
320x240 stream resolution Theora video