Anything that interferes with the flow of direction my eyes take - so yes, bad grammar stands out. As also out-of-context narratives. Topic-jumping. Superficial characters as @cynic mentioned.
Sometimes I am also put off by too long chapters. Shorter chapters provide a necessary break when one wishes to or has to for other reasons. It is less annoying to break the reading at chapter end rather than mid-chapter.
Flip-flop characters, well don't we all just hate 'em? It gets on my nerves when a character start out as something and there after takes a 360° turn. Add to it the flat characters as well. No growth, no change, no arc and the author wants the readers to like them. Like urggh!
I consider the total deviation from plot to be a turn off while starting of the book will have some goal to achieve and going frwd it gets sidelined and sometimes forgotten in some books.
Cardboard characters turn me off big time. The same goes for poor narrative style and an unnecessarily long-winded story.
I don't know about others, but I found 'Illicit Happiness of Other People' way too long. That book could easily have been 50 - 80 pages shorter for a better impact.
Absolutely. I don't need decorative language to lure me. But an impeccable prose with coherent ideas is so needed. Unfortunately a lot of books these days are published just for the heck of it, killing the the very interests.