We put together a survey to figure out what people in the d3 community wanted to learn: https://docs.google.com/forms/d/1Qv2-31Hre8Z9q-SNmmZ6vVyQOgEnKj5_dXNgqp0Ps44/viewform
the results can be obtained in JSON form using the following URL: http://enjalot.github.io/d3surveys/learnd3/results.json