Watch on Youtube:
Research on what makes a quality online TEFL course
It depends on the purpose of the course but for TEFL you need:
Visuals - video and images because people don't remember words they remember images.
Interaction and feedback with a human helps but many online courses contain no interaction which leads to high incompletion rates.
See this page to read some of the research.
ESLinsider's TEKA course contains both.
Search:
teka vs. tefl