Most of our math content creation work these days is driven by our push to cover the new Common Core math standards, which have recently been adopted by 45 states. We're following these standards not only because of their recent popularity but because they are among the best standards we've seen, primarily because of their focus on conceptual understanding. In the last few months, we’ve hired many experienced content creators to work with us on covering all of the the Common Core standards accurately. So far, they’ve created thousands of new handwritten questions that focus more on conceptual understanding to complement the machine-generated exercises we already have.
https://www.khanacademy.org/about/our-content-specialists
All of these questions are going into new exercises, and we're also updating existing exercises when it makes sense. Our new exercises are much more interesting than our old ones; here's one example of an exercise by one of our content creators who joined us full-time this week:
https://www.khanacademy.org/math/trigonometry/functions_and_...
The blog post discusses a few ways in which our decimals content can be improved, all of which are reasonable. Our recent push to improve our math content hasn't yet touched the decimals exercises but it will soon, and we've made a note to make sure that these specific criticisms are addressed, as well as anything else we find.
Most of our content creators have direct teaching experience and we also have multiple people review each new exercise before it's posted, to make sure that it's correct and that it tries to teach students well. In addition to the teachers' own experience, the Common Core progression documents (written by the authors of the standards) have been invaluable. Here's the progression document for the number system for grades K–5:
http://commoncoretools.me/wp-content/uploads/2011/04/ccss_pr...
You can see that on pages 8 and 12 they specifically discuss area models for understanding place value. We already show place values with area for integers; we'll add it to our decimals content soon.
We're making improvements like these on a daily basis, and we're always interested in finding ways to make this process better.
Edit:
One last thing I forgot to mention is our videos. With each new exercise, we're making complementary videos that explain the exact concepts the exercise covers, so our videos are also aligned to the Common Core standards and progressions (albeit indirectly). Even so, our greatest efforts have been spent in improving our exercises because the schools that we work with currently are more invested in our exercises than our videos, and we're looking to help them as much as we can.