These probably aren't annoying to everyone, and obviously aren't things that gets a book rejected from an agent or editor. Most are small issues, and maybe editors miss them completely, or don't see it as a problem. But some of these things drive me right up the wall.
“I'm just here to be pretty” syndrome. This is the absolute worst offense an author can make, in my opinion. I've read books with crappy plot, books with grammatical errors and tons of typos, that didn't piss me off as much as this does. What is it? It's the introduction of a character who we only see described physically. This is always female-apparently there is some innate bias toward describing women in terms of their appearance. The author suddenly goes from a gritty, understated style of writing into a flowery, nearly purple prose style as they describe the female character's beauty, and how she affects the protagonist. Meanwhile, no other character is described physically save for a few sentences here and there. I don't want to name names, but several authors I've read fall into this trap, and it's not only male authors, either. Every female character in their books is described physically, while no male character gets the same treatment. Apparently male characters' personalities and motivations matter, but female characters only need to look good to be seen as interesting characters. This also leads into the next complaint.
“Unnecessary character” syndrome. This is when the author throws in a character that doesn't really serve a purpose, except to somehow make the story seem more well rounded in the author's eyes. These are usually (again...) female characters that are there to be the hero's love interest, and who don't really do anything except serve as some sort of humanizing agent on the main character. Apparently every hero needs a damsel to offset him, but plotwise they have no purpose at all. It's also bad when I see female characters thrown in an all male cast, if only because the author must have felt that the book needed one. If you don't want to write a female character, or your story doesn't need one, then just don't do it!
Fanservice. This term comes from the anime fandom, and refers to scenes in the anime that do nothing and serve no purpose except to titillate the audience. It's usually stuff like panty shots or boob shots. In published novels it usually takes the form of an unnecessary sex scene, or a sudden switch in focus from something really cool, like exploring prehistory, to something incredibly out of place, like a man's version of lesbian romance. Not naming names.
Filler. Another term from the anime fandom, referring to useless scenes. These aren't meant to titillate as much as they are meant to take up space. Published novels typically don't have too much of this, but a few I've seen, especially the longer, bestselling series, have scenes that serve no purpose other than verbal masturbation by the author. Sometimes it takes the form of a ton of detail that no on really cares about, other times it's an action scene or a discussion between two characters that only reiterates something that's already been established. Sometimes it has good intentions, like extra worldbuilding, but if it doesn't advance the plot then it's probably filler.
“Trying too hard to characterize” syndrome. This is when an author doesn't really know how to get their character across the way they want to, and it comes off as forced. Maybe their editor told them their character needed more development or something, but suddenly you have a character using speech patterns that no sane person would, or making decisions that make no sense. They are less a character and more of an archetype, slamming the reader over the head with their “uniqueness.” Most of the time it's characters written by an author of the opposite gender that suffer the most, and in an attempt to break away from cliches the female characters are incredibly tomboyish or masculine to the point that it becomes obvious the author doesn't know how girls really act. On the flipside, the male characters will act either ridiculously aggressive and out of control, or completely effeminate. It can also happen when someone is trying to write a character from a very different background. Most authors who do this are at least trying to create a character, but their lack of experience limits them.
That's all for now. I'll probably think of more in the future. Like with any form of entertainment, the more I sample from it the more cynical and judgmental I get.