In no particular order except what comes to mind:
Booooring backstory dump up front. Even in slow-paced novels, something must be happening other than floating in a sea of atmosphere.
Repeated errors of grammar or usage. Not typos, but clear indications the author and/or editor has no clue about something.
Significant factual errors. I notice these most when they're in my own wheelhouse--the military, weaponry, and basic science (where the science is supposed to be real).
Sudden, explicit/gratuitous sex (or occasionally violence/torture) scenes. I'm no prude--I've written a few of these scenes myself--but they have to be appropriate to the material, the story, and the expectations of the reader. There's one indie author who I won't read anymore--they know who they are, I'm sure, and they sell very well--but they always seem to drop one blatant, pointless sex scene into every book--as in every chapter is PG, to even the lack of profanity, and then suddenly we're sliding fingers into warm wet love-tunnels and caressing throbbing members. (Yes, I did that deliberately--kind of gives you mental whiplash, huh?) Then back to PG for the rest of the book.
Setting up a huge emotional investment in a character only to kill them off. 99% of the time this will backfire, especially if they die pointlessly or too soon. Killing off a beloved character is a huge blow, but it has to be done correctly. Protagonist death near the end of a book or the series in order to save the world or another protagonist can be fabulous. I still get choked up thinking about Mike Havel's heroic King's Death in SM Stirling's Change series, masterfully written.
Conversely, having the reader follow the protag only to have them die in the middle of the book and another protag take over is almost always lose-lose. To me this is an instant DNF. If the death is fake or reversible in some fashion, the reader better have solid foreshadowing of that fact. Just because GRRM made it work, don't assume you can. That's like saying "if Tony Hawk can make that jump, I can too." Most of the time you're gonna splatter. For example, the way the same author, SM Stirling, in the same series, botched Signe's (I think) death scene later on. Ticked me off to no end, and I stopped reading with that book. (There were other problems with that book in the series too, but that issue was major).