What works for Online training?

Hello, I'm working on a system for interactive video learning, and I wanted to find out what other people have found useful or annoying in other online class systems. 

If have you taken or hosted any online training, what worked and what didn't?

The system I'm building so far incorporates numerous small video clips, text material, downloads, etc...  And since it is in construction, it is very open to additional features.

Thanks