I'm going to leave out the obvious spelling/grammar stuff, because obviously that bugs me but it's not so much of an issue in books that have a proper editor (some self published books I've read could really use an editor to fix those things though).
1. Multiple Narrators/Switching POV's
Problems with it:
I usually latch onto one character or one part of the story more than the rest and that is the thing that makes me want to keep reading the book. But when there is like 3 or 4 or even more narrators thrown in there and I need to drag my way through their chapters to get to the parts I want to read? I hate that. And if I put the book down on a chapter narrated by a character I'm not really liking, then I'm much less likely to pick the book up again any time soon.
It can often seem like writers are using it as a crutch because they're not good enough at writing to make the story work without it.
It can come across as really...messy. Like, random POV switches are just thrown in for the sake of it and they really aren't necessary for the plot or even for entertainment value - or they stick out like a sore thumb as being thrown in there because the author wanted the reader to know something and it usually takes away from the mystery or suspence of a book (like in the TV show Pretty Little Liars when there is random scenes thrown in of someone doing something while wearing gloves and we don't see the face and it's just so...facepalm worthy).
If the story can work without the extra narrators, then don't add them, it's that simple. If a book is like 400+ pages and a whole lot of it is stuff that could've easily been edited out without impacting the plot in a negative way? It's shouldn't be there.
There are other reasons I don't like multiple narrators, but my explanation is already too long so I'll leave it at that. Like I said, there is always exceptions, but in general - eugh. Do not want.
2. Accents written phonetically (especially Scottish accents, which non-Scottish writers have a tendency to butcher beyond belief).
It's another thing that really is not necessary at all, so why, why, WHY do some writers feel the need to do it?
If a character has an accent, then say they have an accent - there's no need to treat your readers like morons who need it written out phonetically like little kids learning their ABC's. If you feel you absolutely *must* put emphasis on a characters accent and that the story absolutely cannot work without it then do it but be subtle about it, do it in moderation, don't go over the top - add in some slang words from where they're from or something, don't butcher the English language to try and write phonetically. Just don't.
The reason it bugs me more with Scottish accents is because I am Scottish and authors very rarely get it right. They either go for the cliche version of Scottish accents that people outside of the UK seem to think we have or they will put emphasis on the wrong words and things...like, yes, a lot of us do use the word "aye" instead of yes but we don't say it every single time (honestly, I only use that frequently at home around my family, it's more common for me to say "yeah" than "aye" and if I'm around someone who isn't Scottish then I kind of subconsciously lay off on the Scottish slang).
So yeah. I cannot stand when authors do that. It disrupts the flow of the writing for me and the accents are usually really not needed at all (when I read, I don't make a habit of giving each characters different accents in my head - it's not important).
3. Excessive descriptions of settings.
Some people like this. I just don't. I like some descriptions but there are some authors who go overboard on it, taking like an entire page to describe what a room looks like or a building looks like and it would be okay if the stuff was actually relevant to the story, but it's usually not.
4. Not enough descriptions of characters/no descriptions.
I know I was just whining about too much description...but yeah, I like to know what characters look like. I like to know their hair colour, whether they're girly or grungy or punkish or whatever, I like being able to picture the characters as I read (and it also helps for when I like a book and want to make a fan cast of them). There's some exceptions for this one too, where the book works without the descriptions but in general, I like them included (so long as they're not OTT like the way Stephenie Meyer persistently tries to hammer into the readers mind just how hot Edward is, or how pretty the Cullens are or how oh-so-plain Bella sees herself).
5. Still on the description sort of theme: information dumps.
When a writer will just throw in a load of information in one go and it's so blatantly obvious and doesn't work with the rest of the story. I particularly hate it when it's first person narrative and it's the protagonist decribing themself - like the book Tris and Izzie, I remember the first few pages being just...eugh. It made the character seem so ridiculously full of herself (which contradicted nearly everything she was saying about herself).
Information (about the plot or characters or back stories) can be woven into the story in a subtle way. Like, instead of having the main character look in the mirror and describe exactly how they look (usually in a facepalm-worthy manner) and prattle on about their likes and dislikes and how popular they are and how smart they are, it can be spread out and little bits of information can be woven throughout the story...adding it in all at once is just sloppy writing.
6. I hate it when writers tell instead of show.
There are times when this can work but usually it doesn't. If two characters are best friends or in love, then you should write scenes that show that instead of just having the character say it. If a character is supposed to be smart, then show that they're smart - I hate it when I see a supposed to be straight A student in advanced classes acting like a total brainless bimbo.
7. Writing dialogue without speech marks.
This one pisses me off beyond belief and I'm not sure what the point in it is. I've only forgiven it in a few books that were really awesome, but then, forgiving it is not the same as liking it.
8. Text talk.
I hate it when character IM's or texts are included and they're written in awful text talk. Especially when it's supposed to be intelligent characters who read loads of books and all that...maybe it's just because it bugs me in real life and I only really text with/talk to people online who type properly and I like to think of book-ish people as not typing lyk dis lolz!!11! but yeah...it bugs me. Some wtf's and omg's or even lol's don't bother me but u instead of you and the other ones I mentioned are just - eugh. Irritate me so much. I only tolerate it when the character seems like one of those types.
Also, has to be mentioned, although it's not writing - I hate it when a beautifully written book has a trashy/awful title and cover that doesn't reflect the content at all. Or when there is a beautiful cover on a bloody awful book (e.g. Tris and Izzie).
...I'll leave the pet peeves at that. But yeah, those things bug me in books. Do they annoy you? Or do you have any writing pet peeves?