I'll keep reading a book that's badly written and potentially even enjoy it as long as it's got a great story or great characters to back it up.
I won't. I don't really see how you can have both great characters and lousy writing, though I suppose it all depends on how you define good or bad.
For me, 'bad writing' has to be really bad, not simply mediocre.
By that I mean lazy writing that reads like a rough draft where:
- the author doesn't appear to have cracked open a thesaurus or even a dictionary for that matter,
- each sentence is the same length or rhythm so it feels you're being banged about the head,
- the dialogue is embarrassingly trite or corny or reads like a 60s sci-fi B-movie,
- the characters have really ridiculous names that in real life would send CPS after the parents, and
- serious errors in building up to the main 'turning point'*
- idiotic or annoying characters that are well-loved by the MC for some reason that's never made clear*
- critical plot points or dramatic scenes are expository instead of being written into action and dialogue.
However, it does take several repeated violations of the above, not just a handful.
* I put down one recently where these were the only problems. The book had a lot of potential but the entire plot hinged on the MC finding a close friend who had been abducted and she was introduced at about page 60 or something at least, immediately before the character went off to find her. If she's that important, there really should have been some buildup first - as it was, I didnt' give a crap about her and thought the MC was an idiot to risk his life for her.
- The MC's girlfriend was also a shadow - the only description of her was that she watched 'Real Housewives'. Ugh. At least give her really beautiful eyes or large breasts or an exciting career.
Something.
What I'm much less forgiving of as a reader is story flaws. Plot holes, moments that break suspension of disbelief, characters doing things that are good for the plot but make absolutely no sense for the character, that sort of thing.
See, for me that IS bad writing as well...