Long descriptive passages. Recently I've been reading a lot of John Barnes (sci fi), and his worlds are really well developed. I like him a lot. But then sometimes he goes overboard. Like describing in elaborate detail: pseudo-Mayan temples, city block layouts, smart snowshelter functions, futuristic technology, etc. etc. Frankly, I can imagine what a Mayan temple looks like, I don't need two pages of intricate details. I end up skipping to the next line of dialogue.
Another boredom/irritation inducer... I won't name this author because I find his writing atrocious and I confess that I only read him because 1) He is fairly entertaining and simple and 2) his prose is so awkward it makes me feel like I'm awesome. Anyway, my major beef with this guy is that he uses ALMOST the right word. A few examples:
"I've never known him to be so damaged by the death of a friend."
"dying torches tossed their light onto the men"
"a stream of tears burst from beneath the lids" (of his eyes)
"he could spot a red-robed [Orc equivalent] faster than a hawk"
This is all from the first few pages, including the unnecessary prologue. I mean, you can become damaged by the death of a friend, but it's not something instantaneous... I think he means "upset" or "disillusioned." And how does a torch "toss" light? Does he mean "cast?" I've never seen tears "burst" from anyone's eyes, except maybe in anime. And are hawks really known for how FAST they can see something? No, it's the detail with which they can see. Constant little things like this just drive me crazy.
Also, this has nothing to do with the writing of a novel itself, but it often determines whether or not I read a book: cover material. This includes design, artwork, and back cover text. I have high expectations for all of these elements because they are the first thing you see-- ergo I assume a great deal of attention has been paid to putting them together. What I really hate are generic cover pictures. For sci fi, a spaceship and/or a field of stars. Do publishers just have a stock of space paintings they assign, or what? For other stuff, either generic designs or classic paintings that have nothing to do with the work. I want the picture to SAY something about what's inside.
As for back-cover text, if it 1) tells me how much I'm going to love this book, or 2) explains the setup of the world (ie "in a future where women are extinct, men have to procreate by blah blah..."), that's an immediate no for me. In general, I want the back cover to introduce me to the main character and give me a reason to care about this guy/girl.