Completely agree. As someone who has zero knowledge in typography, it would have been nice to get some explanations as to why Arial and Helvetica are so horrible.
The exaggerated x-height — the height of the 'body' of each letter compared to the height of the ascenders and descenders (the stem of the 'h' is an ascender, the letter 'x' has no ascenders or descenders, hence x-height) — the narrowness of the typeface, and the (relative) lack of distinction between letters, makes the letters harder to distinguish in large bodies of text, increasing cognitive load (your brain needs to work harder to decipher the glyph), and thus reducing readability.
