The entire success of Khan Academy rests on providing simple quick introductions to different topics. If Khan made the videos longer or went into more details the students would get bored and it might as well be a History channel documentary or a NOVA science show (fun for certain people but unlikely to capture the interest of many 8th graders).
As to this specific video I actually thought it was very good - precisely because of Khan's conversational breezy style that managed to keep my interest even though I was quite familiar with these events. A longer lecture would have me closing the tab pretty quick and I imagine the same reaction from the target audience.
Lastly, Khan Academy videos in their current form are probably not going to replace the entire educational system, but if you want to get a quick introduction to a certain topic they seem to be effective. For example I just watched this video a few minutes ago and approximately doubled my knowledge of diabetes: http://www.youtube.com/watch?v=rPLjSY00JlE and I would guess some of the viewers have similar increases in their knowledge in other topics. THAT is where Khan Academy currently excels and it is a perfectly worthwhile contribution.
As to this specific video I actually thought it was very good - precisely because of Khan's conversational breezy style that managed to keep my interest even though I was quite familiar with these events. A longer lecture would have me closing the tab pretty quick and I imagine the same reaction from the target audience.
Lastly, Khan Academy videos in their current form are probably not going to replace the entire educational system, but if you want to get a quick introduction to a certain topic they seem to be effective. For example I just watched this video a few minutes ago and approximately doubled my knowledge of diabetes: http://www.youtube.com/watch?v=rPLjSY00JlE and I would guess some of the viewers have similar increases in their knowledge in other topics. THAT is where Khan Academy currently excels and it is a perfectly worthwhile contribution.