I can get put off by really bad grammar but generally my feeling is that grammar is just the clothing which wraps the story and if the story is strong and beautiful I don't care about the clothing too much.
Sterile description and stilted unrealistic conversation can get to me too unless the story is gripping. Over floury or complex language can be a turn off too as well as stories which just don't 'scan'. But again all this just makes a story harder to read and makes me try less hard to like it.
I would say the one biggest turn off for me is that there is no basic story, nothing happening, nothing building no characterisation, nothing to care about or look forward to.
There are stories on GA and everywhere, even books on the shelves, which are well written with perfect grammar, reasonable dialogue and an okay 'flow' but go nowhere. They are... frankly boring.