Videorecording lectures is like filming a stage play (or a concert). An excellent performance can cross the media difference, but the odds are stacked against it. In order to make the recording snap, one needs to render the performance on the medium’s own terms. Compare a static camera recording a rock concert with Jonathan Demme’s Stop Making Sense, for instance.
(I know I’m not the first to say this — just marking it down here since it crossed my mind again this morning, and by writing it out I may prepare my thoughts to move on to the next topic.)