- Information density. This is especially a problem with communication tools, which for some reason are all copying Slack and leaving very little screen space for the thing you actually want to see - the text that the other person is writing. Messages go off the screen very quickly and a lot of screen space is spent on decorations (user icons, names, speech bubbles, padding) leaving it with basically the density of comic chat. Graphic background images with semitransparent visual elements on top are another similar trend which uses a lot of padding to not obscure the background too much, and the transparency makes contrast and readability worse.
- Affordance discoverability. Many modern user interfaces have a lot of functionality hidden behind catchall elements like a user icon, a gear icon, or a hamburger menu. This may be a misguided attempt to make the user interface "cleaner" but it makes it very difficult to find what functionality is available unless you go exploring. This is especially bad for context-dependent functionality because there's no visual indication that the hidden contents of the catchall have changed.
- Delineated targets. A lot of modern designs have flatness as a design concept, meaning the click or tap targets are not clearly separated from the static parts of the interface. This makes it hard to know that they are targets (as it's common to have nonclickable decorative graphics too) and hard to tell where exactly they start/end so it causes extra cognitive load in order to hit them precisely.
- Non-gigantic targets. This is related to the low information density mentioned above. More and more designs target smartphone displays, which used to be very small and require big targets to ensure they could be mashed with normal human fingers. However, this gets reused on tablets, modern gigantic smartphones, and even desktop, where none of this makes sense.
- High-contrast targets with identifiable icons. Low-contrast or monochrome targets have become fashionable. Clickable items often have simplified, highly abstracted icons with thin-line monochrome content, frequently implemented as fonts, which is less distracting from the visuals of the design but is also hard to understand what it's supposed to represent. Another common one is having said elements in grey on grey, making them very difficult to read for people with less than perfect vision.
- Less modality. Many modern designs will have modal interface elements that prevent access to the rest of the user interface until interacted with and/or dismissed. The "blur the content and put a tiny dialog with a huge button in the middle" is a common tactic, and is often user-hostile nudging trying to get the user to do something they don't really want in order to get rid of the thing that is literally blocking their access to whatever they were actually doing.
This is basic human-computer interaction design stuff, but somehow it's all gotten lost.