I lament the death of always-visible, easily-grabbed scrollbars. Every scrollbar is transparent, disappearing, tiny, and exceedingly difficult to grab. Why? I guess UI designers decided that scrollbars are ugly. Same with obvious buttons; at some point, half of buttons were made into obscure symbols, and even when words are still used, any kind of bordering that points them out as buttons is gone. Just freestanding symbols and words. It's infuriating.

Low-contrast colors EVERYWHERE also angers me. Low contrast is great for being easy on the eyes, but certain interactive elements need contrast to indicate their function. My best example is that Linux desktop themes across the entire DE space focus on low contrast at the expense of almost everything else, and it's hard to figure out stuff as simple as which window is on top and active, or which elements in a window are clickable.