Perhaps the largest repository of screencast lectures is found at the Khan Academy. The library of topics covered seems to be growing exponentially. What started as a site to teach mathematics has grown to something that provides insight into sociology, biology, economics, politics and more. The growth of the site is linked to the flow of resources into it from donors who recognize the great work it has accomplished within this type of content delivery.
The success of the Khan Academy is due to its success in presenting information. People would not be tuning into and returning to the site if they did not learn something from it. For us as screencasters, it is also important for us to learn from this success. To that extent view this video from Salman Khan about his screencasting technique.