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.
Best Answer
Readability is not simply "being able to read each word", but also being able to understand the relationships between the words, between the paragraphs and so on.
Titles are put into evidence because that's their role: stick out as much as necessary for the reader to spot them and know that something has changed. Paragraphs are indented for the same reason and lists are given a different treatment because they are different from normal paragraphs. The interline leading is not too wide so that the eye can catch easily the next start.
Here's an example where some of the "rules" above are broken: one can probably read that text, but not as fluently as if it were well presented.