I was recently handed a book to read that the person who gave it to me thought I'd really enjoy. From the snippet on the back cover, I agreed that it sounded like something right up my alley. I'm not going to name the book, but it was set in Seventeenth Century Scotland, and is chock full of mysticism. I read about forty pages before I set the book aside, and started my decision on whether to finish the 400 or so pages or not.
When reading a book, there are certain things that I can overlook if the book is overall enjoyable. If the world-building is lackluster, or the character development isn't as strong as it could be, those are generally things I'll look past. If the writing isn't so great but it's still an enjoyable story I'll generally keep truckin' along. The one thing that tends to be a deal breaker for me though, is the dialogue.
The issue that I had with this book, and which I find mostly happens with the historical fiction genre, is that the dialogue is not believable. People spoke differently in the 1700's so if you're reading a book and the characters talk like we do today, it makes it hard to take seriously. A great example of dialogue fitting perfectly in a historical fiction novel would be Diana Gabaldon's Outlander series. She writes some of the best dialogue I've read.
Sometimes it gets hard as well when after each dialogue the author writes, 'She said.' or 'He said.' In my opinion, this is the chance for the author to really relay the emotion behind the character's words. If two characters are fighting, it's better for the dialogue to be spoken and then say something like, 'She screamed.' followed by maybe, 'as her face turned red and she stalked the length of the room, anger swallowing her up.' Again, I am not an author (writer, yes. Author, no.) so this is just my personal opinion.
Is there something that you tend to consider a 'deal breaker' when reading?