I did just test turning off webfonts on Medium and I still honestly can't see any huge problem with my default system typeface. This could possibly be because I have nice defaults, but at the same time, I think the good readability of Medium is far more about the overall style/layout of the page (particularly the decent large font-size), than about any particular typeface.
Example 1 - normal Medium: http://i.imgur.com/D4hYBPy.png
Example 2 - default system fonts: http://i.imgur.com/Jvnuhkm.png
Whilst I can see the difference in typeface, I can't honestly say that I'm experiencing any huge readability changes. Perhaps if I were to switch to a Sans of some sort, but that's not exactly a huge decision that needs to take a lot of time and fuss. By far and away the most important thing that makes this page readable and nice is the font size, closely followed by the line-width.
http://books.google.com/books/about/The_elements_of_typograp...
for a discussion of LOD in typography from a master designer)
To properly design his multi-volume set of CS/mathematics books:
http://www-cs-faculty.stanford.edu/~uno/taocp.html
Knuth developed an entire software toolchain that spans type face creation (metafont) to the document layout programming language TeX (yup, it's an actual Turing complete language!):
http://en.m.wikipedia.org/wiki/TeX http://dl.acm.org/citation.cfm?id=67555
He is also responsible for the creation of "literate programming"... All this in his spare time, of course, while he wasn't occupied with his responsibilities as a CS professor at Stanford, teaching, doing research (on things other than digital typography, e.g. the content of his books, combinatorics, etc. (...). Truly an inspiration!