I was able to read only part 1: Foundations, and these are my impressions. Definitely goes into my Bookmarks, so that I can read the rest...
Edit: The title was changed from "Probability Theory 101 (by Fields medallist Terry Tao)".
While the novice programmer might not get (the content and purpose of) python vs. ruby comparisons and the like, readers with a weak background in math will not find this probability entry useful (or too 'difficult').
However, for a certain subset of all hacker news readers, which are looking to dig deeper, it is a very nice resource.
Hacker news, to me, is especially useful since it provides links which are well distributed in terms of this 'entry level'.
Perhaps something more visual and involving interactivity (like visual programming).
What percentage of students after having passed a probability course can define a random variable?
In a typical probability theory course, all of them, hopefully. I found random variables to be a crucial part of my probability theory class, being used in almost all (if not all) topics after the first month.
There are certainly other approaches to learning probability theory (though perhaps not what you were thinking.) I understand that E.T. Jaynes' book on probability theory, which I have not yet read, does not go the route of random variables.
They may use random variables frequently without being able to define them.
Jaynes' book is excellent but is more about the philosophy of the Bayesian interpretation of probability than mathematics.
His problem with "random variables" is not so much the concept (although he wasn't a fan of measure theory), but the "random" in the name.
This blog entry is meant for a review for a graduate level course, so it makes sense that it runs through a lot of formal definitions.