There's an immediate and obvious answer for me that one other person pointed out: The index. This is the most god-awful index I've ever seen in a book, and it goes out of its way to be as obnoxious as possible. It's been a joke for the past ~4 years that any time someone opens the index to find something, it's going to say "See X instead", because that's more helpful and less wording than saying "p. 292".
I really don't like the very odd quirk where skills are listed as Attribute (Skill Name). That breaks with previous editions and doesn't make a lot of sense to me. It should be Roll an Athletics check. If you need a stat that's not the norm, such as a Constitution-based Athletics, a Wisdom-based Nature, Charisma-Based Investigation (we use that to replace the defunct Streetwise), then say "Roll an Athletics (Constitution) check".
You can add in moving the Open Locks/Disarm Traps off the skill list and into the nebulous Tools, which 90+% of the time are Thieves Tools. At the least, there should have been a few Fill-in-the-blank "X Tool" listings on the skill section. This causes so much confusion explaining to newer players, who are often attracted to the Rogue class.
Here’s another pet peeve for me: no indication of expected challenge for an encounter in the published adventures. Not even the CR! Making us all have to reverse engineer each one to figure out if it needs adjustment for our table.

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I've said it before, but I wish they would at least list the CR of all of the creatures in the books. If I see that there's three NPCs in a fight, and they're all CR 2 against my 5th level party, I can look closer but assume that it's probably a fair challenge for my PCs. If they're all CR 1/2, I might think that's probably not enough and look closer at the number of additional fighting encounters to see if this one needs buffed up a bit.
From a personal example, one of the biggest places this bit me in the butt was in Curse of Strahd when they were investigating the winery. The recommended level was 5, but my PCs got there at level 4. Ok, so it may be a bit rough. The very first (likely) encounter is if they are seen approaching the winery, and 30 Needle Blights attack them, with waves of Druids coming out in subsequent rounds. I figured the Needle Blights were something on the order of goblins, whereas they were actually more like 1st level Rangers with decent hit points and ranged attacks. If I had seen their 1/2 CR, I might have given it a second look.