This is a nice tutorial. I like the order and accessible way it introduces selections and data binding, which are some of the harder things to get into when starting with d3.
However, once you dive a bit deeper with selections and nesting, there are some things that are not immediately obvious how to structure. For example, when we implemented small multiple visualization, figuring out the best way to structure enter and update sets for nested data took a few attempts.
Sadly, getting around to properly explaining them will take another lesson or two.
PS: it's a fantastic tutorial, thanks for writing it!
This is another must-read: http://bost.ocks.org/mike/join/