Forget the videos for a moment. Khan Academy has, below every video, a collection of student questions on the material. To be fair, the questions are mostly irrelevant. But there a ton of gems buried in there, and Khan Academy is definitely a place that I'm going to stop when I'm trying to figure out where my students are going to trip up on a topic. A lot of these questions are going to make their way onto the Math Mistakes blog eventually, but for now, here's a sampling.
Sorry for the small picture size. Click on 'em though -- it'll only take a second.
The thing that gets me most excited are the questions on the primary school material. One of my weaknesses as a teacher is my inability to effectively help students who are struggling with negative numbers, fractions, multiplication and division of fractions, square roots and the whole array of stuff that kids are supposed to know coming into my classrooms but don't. And the blogging/tweeting community skews towards teaching older kids, so I have fewer folks to lean on.
Digging into what kids struggle with in earlier math material is proving to be super interesting (of COURSE kids would think that a negative plus a negative should be a positive) and I'm betting that it'll make me a better teacher.