Can't believe someone hasn't written this already: fonts. I've used Debian and Arch and had massive trouble a) figuring out what fonts are installed, b) how to install new fonts, c) getting them to not look like absolute shit. Example case: trying to use Adobe's "Source Code Pro" font with Debian's default terminal, at around 12-14px, on a 2560x1440 screen. I could get it to look good at 10px, and at 18px, but nowhere in between. One of the most frustrating experiences ever. To this day my font size is smaller than I'd like.

Another example: any font rendering in a web browser. Generally terrible.