I was actually thinking of writing a piece about the legibility of typefaces in the context of UI. It would have gone far beyond for this post though. In the meantime, go to the website of Monotype and write the word “Iliad”, once in Neue Haas Unica (similar to the Apple font) and once in FF Unit. Notice how you can tell apart the capital “I” from the lowercase “l” and “i” much better. It’s things like this that make typefaces like FF Unit much better for UIs and also signage (in hospitals for example).

I would love to read your (future) piece about legibility of typefaces!

You're correctly describing facts (eg the font apple uses doesn't let me tell the difference between llama and Ilama), but that's mostly not needed when reading their blob of legalese text.

In the extremely subjective world, I find the monotype hard to read in that narrow format because it implies a verticality - like looking at an art deco building. Somehow my eyes are able to scan the font they've selected more easily. This is, of course, possibly because apple has made me read a lot of their typefaces.

The context matters a lot, and so you want the stop sign (or possibly even the simple marketing sign) to be unambiguous. You want good kerning that won't make words bleed together or break at incorrect points.

I tend to like the fonts that are trendy on the web these days though - I find them fairly easy to read (and I read them a lot!)

One thing that surprised me is that comic sans can be easier for someone who is dyslexic to read because b and d are not mirrors. So I would be sympathetic to a lot of typeface choices if I understand why.