I'm typically the type of person who finishes a book no matter what. I'll slug through the slow ones and cringe at the poorly-written ones. Why? Probably because I'm a writer and I know how much heart writers pour into their writing. The other reason is that I'm a little bit of a perfectionist about my reading habits. When I put down a book for good, it's straight unsatisfying. And I'm stubborn.
I've gotten way better at this, though, and will occasionally put down books I find not only unenjoyable but BAD. Without naming names, I'm going to explain what--and why--I'll put down a book.
Repetitive writing style
Bad writing can ruin the whole book for me. I don't care how good the story is, if the writing sucks, I'll stop reading. A couple years ago, there was a book with an absolutely riveting plotline--unlike anything I'd ever read--but the writing was so darn repetitive. There was very little variation in sentence length and structure, which made it very tiring to read.
Clumsy writing style
I recently put down a book because I detested the writing. It lacked flow and nuance. What I think contributed to this was the third person narration. Now, there's nothing inherently wrong with third-person point of view, obviously, but this perspective lends itself to too much telling and not enough showing (more on that later). Because of this, I felt quite distant from the characters. The writing got in the way of me connecting with the characters. After fifty pages (which is when I gave up), I cared nothing for the characters at all. I wasn't even remotely intrigued by them. That's bad.
Not only that, but the author relied a little too much on sounding up-to-date on social media features without stopping to consider whether or not it would sound outdated in a year or two. Maybe I'm too old-fashioned, but I feel it's best to keep mention of technology and internet culture to a minimum--unless, of course, you're writing historical fiction or a book that directly critiques internet culture. That's just my opinion, though.
Too much explaining
On top of the clunky writing, the writer was giving away everything. The nuances of the characters were not developed organically, and I don't like how the author revealed backstory. He did it in a way that interrupted the plot, rather than fed us breadcrumbs through dialogue and description.
He also did it way too early. When I first meet a character, I don't care about him. Nothing about him has interested me yet. If I don't care now, why should I care about what happened to him before the book started? You need to flirt with me before you tell me your whole life story. That's too much too fast.
Passive characters
This one's pretty self-explanatory and probably the biggest reason readers will put down a book, myself included. In order to create a compelling character, he needs to want something. And there need to be consequences if he doesn't get it. If the author fails to set up these stakes, I'm closing the book.
Boring characters
Sometimes, though, characters are just plain boring. There's nothing that specifically defines them. If you're wondering whether or not you've written a boring character, try this test: find three adjectives that describe your character. If you can think of three words, think of examples from your draft that exhibits these qualities. If you're drawing a blank, you probably need to do some more fleshing out.
Wooden dialogue
It may be difficult to describe why dialogue doesn't sound quite right, and we might not even realize it on a conscious level. Heck, we might not even sniff out that dialogue is the thing weighing down the whole story. What I think it comes down to is this: do people really talk like that?
When we talk, we often leave out words, smash them together (i.e. contractions), or string together our thoughts with fragments. I'm not saying every line of dialogue needs to sound rudimentary, but read it out loud and see if it sounds like it would come out of someone's mouth.
Even if dialogue sounds decent, it's often riddled with too much exposition. Writers try to get away with revealing backstory through dialogue rather than narration, but it still doesn't work. I mean, it can, but you need to use it in moderation, and you need to consider the context. If two characters are talking, for example, about something they've already talked about a handful of times, why would they recap those previous conversations before saying anything new? Whenever you walk in on two people having a conversation, you have to figure out by inference what they're talking about. They don't just conveniently explain the context.
The lack of conflict
This is another easy one: nothing happens. Maybe you've sufficiently set up your characters. Maybe you fully entrenched your readers in your world. But if there's nothing happening--if the protagonist wants nothing and has nothing at stake--then I'll get bored. Writers shouldn't rely solely on the characters and their world being interesting.
Not my type
This doesn't have anything to do with a book being bad necessarily, but sometimes a book is not my cup of tea. I'm not jiving with the writing style, the characters, or anything that makes the book unique. I'll usually read the blurb to see if the book is my type, but sometimes I pick up the book anyway and it doesn't meet my expectations. But that's okay.