Khan Academy does exactly what you described. Each video is short and on a
very specific topic. The videos are also just one voice drawing at a board
to simulate a personal tutoring session. The key is that Salman Khan is
good at explaining math so it makes sense to tap experts who could explain
a topic clearly and concisely. The illusion of a 1:1 session I believe is
key too.
If you have not seen the Khan's Ted Talk, it's provides good food for
thought. He talks about classroom flipping and how it supports
differentiated learning.
http://www.ted.com/talks/salman_khan_let_s_use_video_to_reinvent_education.html
Kevin Gaugler
Associate Professor of Spanish
Marist College