LaTeX makes things more legible by following certain rules, and there is a strong emphasis among LaTeX users on obeying typographical conventions to make things more legible.
What happens when we break those? Can you give examples that demonstrate why we have certain typographic conventions by providing counter examples? i.e. if we run the following code we can see that the text is still legible, but much harder to read. I think this would be a fun & easy way to learn why we have them instead of just saying "Do this, don't do this"
For example, I use a font called ProFontWin. I use it for programming as you never mix letters up with one another, but it is terrible to read body text in as the eye doesn't flow over it. Similarly using a monospaced font makes each letter easier to see, but the text harder to read. These are both typeface examples: What are some typesetting examples? Preferably with horrible, terrible, no good, very bad, counter examples.
The most obvious one I can think of is spacing: If you changed the spacing between words it would be harder to follow the text (Example: Justify anything in MS Word)
The story behind the question: I recently had to jump through a bunch of hoops due to changing regulations at my university, and for a few moments contemplated turning in one of the required documents in the most illegible typeface I had installed (Blackadder ITC, Old English Text MT or something), so that it was technically handed in, but really annoying to read. I decided annoying the department secretaries (aka the real power in the department) would be a Bad Idea, but this question came out of it.