It's when they pop out of nowhere completely at odds with the setting of the quest/dungeon/no rhyme or reason to them. Quite often there aren't even any visual clues.
I agree with you. (And to some extend, with the OP).
I don't mind if the game is hard. I don't mind if some quests or some caves are hard.
The problem is, I have no idea what I'm getting into. An NPC gives me a quest "go get me this book". So I go there, it is in a chest outdoors. Just open the quest, done. Or you are sent to a cave, kill a few mobs, get the book. Done. But then you get a similar quest, just "get me another book". I'm at the same level, it's the same questgiver, same wording, looks like the same type of quest. But when I enter the cave, bam, no chance. Getting owned left and right by every mob. If I had known I wasn't ready for this quest, I'd done something else first.
Or worse. You enter a cave. Kill a bunch of NPCs. Continue while slaughtering NPCs. And suddenly, bam, some regular NPC kills you over and over again. Not a named one, not the end boss of the cave, just a regular one. Sometimes with a new name you hadn't seen before. But I've also ran into a regular bandit that hit like an endboss. Very weird.
Or a quest to go somewhere in a cave. In the first room, there are 2 snowy sabre cats. I can't even kill one. Let alone two at the same time. Why was I sent there ? How can I know which quests are totally out of my league ?
My questbook is exploding. And every time I turn in a quest, I get 2 new ones. I can't speak to NPCs anywhere, or they trick me into taking another quest. No problem in itself. But it would be nice if I knew how to clean out my questbook systematically. By doing easier quests first. But with the current questbook, I'm just randomly trying quests, and hoping they aren't out of my leaugue still. That can get a bit frustrating sometimes.
Unrelated to the OP's post. But related to the questbook. I wish we could see more info about each quest. Especially the ones under "miscellaneous". Seeing only a single line is sometimes not enough to make me remember what exactly it was. Or who gave me that quest. Or why. Or where the questgiver was located. Skyrim is an RPG, but it is also a story-telling game. The quests tell little stories. With just a brief questbook, I feel I'm missing out on some of those stories. Especially when your questbook is so full, you can't remember everything by heart.