I'm really excited by what Khan Academy is doing, and I think it's great that content is available for all skill levels.  However, I find it difficult to find content that is specific to my skill level, not because it isn't there, but because I'm not assessed by Khan Academy like a teacher in a classroom would, but I'm instead given a curriculum to follow.  In many subjects, such as linear motion for example, if a person understands the basic concepts already they can still continue to learn more about the subject on a detailed level after they've learned the basics.  I find many times the topics go over aspects which I know already, and I find myself not being engaged or feeling like I'm wasting my time.

It seems this would work great for teachers and students, the former who design and the latter who are following a specific area of learning to follow and hope to measure their achievement in that subject area.  But I am looking to continue to assess my skills and expand my knowledge, and the current format which is formidable in its scope somehow fails to connect with me.  What I have to compare it to is Vocabulary.com, which I use & go back to from time to time.  It assesses your skill level, which gives you a sense of achievement before you begin (if you've already spent time on a subject).  Further, you continue to achieve, and you have several choices for learning new words (definition, "word in the wild", etc).  This is only one subject, vocabulary, not a range of concepts, but I'm sure even a simple assessment of a skill level across categories could help make the content more engaging by directing a person to the right videos before 5 minutes have gone by and they haven't learned anything they haven't heard before.