I hadn't seen this topic before, and I find the responses interesting - and quite diverse. Which means you're never going to please everyone.
So, as a writer, be sure to please yourself first.
But for myself as a reader:
In cost/benefit terms, it takes a lot of plot benefit to overcome bad-grammar costs. Run-on and poorly-organized sentences send me away immediately.
Punctuation counts, and as much complaint as there is about the English language, the punctuation rules aren't that difficult to understand. For me as a reader, they don't have to be followed religiously, but the exceptions should be few and justifiable. I once went to a restaurant where every plural had an apostrophe in front of the terminal "s". Unfortunately, that spelled out the quality of the meal, too.
As to the rest of grammar, the internet provides wonderful resources. And in spite of the value of grammar detection tools, good luck. To my own experience, even the best have serious faults. Every author needs an anal-retentive editor and preferably one or two beta readers. A grammar tool and one set of eyes isn't enough.
An info-dump ends my interest. If it isn't critical to the plot, I would rather decide for myself why the guy is "hot" or "sexy". Let the readers exercise their own imagination, please.
There's a fiction that circulates from time to time that the first chapter must introduce the main characters. I prefer the first chapter to grab my attention, and if some main characters don't show up for a few chapters, that's fine with me.
I don't care for excessive detail. It only takes a sentence or two and a few well-chosen words to establish the environment, physical and/or emotional. Don't flog it to death. And individual relationships between each and every character expand exponentially, don't always matter, and bloat an otherwise-interesting story.
I don't care if dialogue isn't written in the Queen's English. People don't speak that way, so why pretend that they do? On the other hand, about the third time an overworked cliche is used, I leave, just like in real life.
I read several genres, and understand that some leap beyond earthly constraints of gravity, time, and space. But even with that freedom, the story still has to make sense, if only within the bounds the author sets.
So, thanks for the opportunity to express my thoughts on the topic. Getting them out of my brain helps me understand them better.