Have you ever put a book down because the main character was being stupid? I have - more than once. It's the sort of thing where I am going along, and then I see the main character make some decision that will bring on a lot of trouble, and I don't understand why he/she is doing it, and I can't put together any kind of mental logic for the character that will cause it to make sense. If it's the first time the character has done this, I won't throw the book across the room. But if it's too important to the plot, or if I've seen it happen more than once, I will. At very least, I'll set the thing down, at the risk of never picking it back up again.
So what is it that gives me this reaction? Well, I'll tell you now, it's not just characters with a low level of intelligence. I have been thinking about this for a while, and despite the fact that I enjoy intellectually sharp characters, I don't require them to continue reading. At one point I even asked myself in adversarial fashion, "What, Juliette, are you really looking for characters who are exactly at your level of intelligence and can't make mistakes?"
I'm not. What I'm looking for is a character whose level of intelligence I can predict reliably.
I'm okay with characters who have knowledge I don't have. Particle physicists are allowed to be particle physicists. But I do expect their mental resources to coexist with a predictable level of general common sense, and for those mental resources to be affected by the physical stresses of their surroundings. If a woman who has studied for years in a magic school suddenly makes a stupid beginner mistake with her magic, I would really like to know why. Does she have a knife in her leg? That could be a good reason. Has she been startled? That's possible. If she's sitting in a place of calm, though, and I can't see any reason for the mistake, I'm going to be skeptical.
I recently read Watership Down by Richard Adams. It's a story about rabbits - real rabbits. And one of the things that I find wonderful and brilliant about it is that the rabbits are not hyper-intelligent. They think things through the way rabbits think things through. Only one of them comes up with really innovative ideas, like having a couple of the smaller, weaker members float across a brook on a piece of board. The others are not capable of this, and in fact can't comprehend it even when they see it happening. A quote: "Frith and Inlé, they're sitting on the water! Why don't they sink?"
This is not frustrating. It is delightful.
One of the things I enjoy most when I'm writing is to make characters whose reactions wouldn't be what we'd predict from our own experience, but which make complete sense within their own worldviews and levels of intelligence. For example, my servant caste character, Aloran, is highly trained and believes firmly in the vocation of service, which is to say that he believes in selflessness and in putting his mistress' concerns before his own. He and his mistress have a number of interactions where she shows consideration for him - and though he's grateful for her consideration, he also thinks that it is at times inappropriate, as when she puts herself in danger of reprisal for defending him. In the situation, he internally urges her to stand her ground against her abusive husband. However, when the interaction is over he realizes that her bravery is going to have horrible consequences - and he bows to her and says, "I beg you, please don't consider me again." It isn't what we'd do, it's what he would do - and because we know him, we can see that it is what he would do.
To my mind, this is one of the most important things you can do for a story - actually think through your characters' level of intelligence, their experience, their knowledge, and their motivations. If your character is well-grounded, you can get away with all kinds of things. The unexpected isn't frustrating, it's delightful - because you've done the work to make sure that it grows naturally out of your character's established mentality and their character. In fact, this is more important than the needs of the plot. If the plot needs something and your character isn't providing it naturally, don't force him/her. Go back and change the character so that person will provide it naturally. Give that person higher intelligence. Or if necessary, lower intelligence. Or change circumstances so an intelligent person is under enormous stress. Make sure that the character's actions fit with his/her backstory and general attitudes, or at very least show us the reasoning that makes this particular decision understandable.
It's something to think about.