I review the lessons on the command line. These introduce the basic concepts of driving the command line, not going as advanced as scripting or if/fi constructs, etc. They are presumably targeted for someone who can get to Terminal or Cygwin, but then who gets rapidly bewildered.
For the introduction, I can envision a pretty good high-production-style video that could be made: showing a typewriter, etc., in a running commentary almost like a TV commercial or Bill Nye the Science Guy. This would give the opportunity to use visual analogies as well, like a transmission line or translator (for the shell between the users).
The content is in good shape, and the slides work well as slides (if slides are the way to go). I would add a one-page summary (here’s what you learned) at the end, perhaps in PDF format. (There is some of that, but I’d expand it further.) I would also rework the material with exercises as follows:
The best way to learn is by doing, which means you need to get the users opening a terminal and following along as best they can while the tutorial is ongoing. Some aspects can’t be easily illustrated this way, but a sandbox tool (in Flash or Java or a Python wrapper) would be interesting. Find a way to make this tutorial interactive: you type in something, then get the commentary as a layer over the work you’re doing, or next to it in a separate pane. That could involve bringing in the aforementioned sandbox tool to the browser; alternatively, it could mean pulling the lesson down and making it a standalone program. Then it’s more like the tutorial level on an RPG or RTS game—it lets you play safely, but still get a feel for the mechanics. So a shell which gives access or emulates the main shell, but sharply limits you to focus on the task at hand and contextualizes the commentary (Figures 1–3).
This could primarily task-driven (like the tutorial) or it could include challenges which force the user to stretch him- or herself. (Kind of like the Project Euler tasks.) This also suggests using Python notebooks in some of the other sections as well.
<figcaption class="wp-caption-text">A hypothetical sandbox interaction for the third shell lesson.</figcaption></figure> 
<figcaption class="wp-caption-text">A possible challenge task for an intermediate beginner.</figcaption>
<figcaption class="wp-caption-text">A hypothetical sandbox interaction for the ninth shell lesson on ssh.</figcaption>