The easiest way to write a bad book is to start with the assumption that readers are stupid and need everything explained to them.
In slush piles, I see many examples of this. Whether it's a sci-fi book that tries to explain every detail of how everything works or a children's book that tries to keep everything happy and fun, it never works.
What readers want, including me, is books that aren't afraid of the scenarios they present and give them in simple and immediate terms.
Recently, I've been rereading an old favorite series from my childhood, The Animorphs. There’s a lot of heavy issues in these books and they aren't afraid to let traumatic and bad things happen. Since these books are for kids, they obviously leave out graphic violence, but they aren't afraid to deal with a lot of the very real emotions or philosophical problems that might happen in the possible world the book presents.
A book treats readers as intelligent when tools, devices, and issues are presented as they would be thought of by the characters (or perhaps the narrator in some books). For example, when I get in my car I rarely consider how everything operates unless some specific aspect of my car seems to be malfunctioning. Usually I’m happy to think of my car as simply the way of getting places rather than as a collection of parts operating through certain laws of physics and combustion to propel me somewhere.
What matters more is that my car works and that I’m going somewhere that feels fun or important. It’s the same for the characters in books. They need direction and some goal to move towards rather than a bunch of explanations. They need to spend time thinking about what stands out to them in their worlds, lives, and relationships, rather than spend any time thinking about details that don't matter to them at all.
Smart writing creates the focused characters and plots that make for interesting books. And those are the books we all look forward to reading.