I'll forgive a few misplaced commas.
But a misplaced comma can alter the whole meaning of a sentence. The person writing the sentence knows what he/she is trying to say, so it may not seem like such a big deal, but if the reader says "Huh?" and has to go back and reread the sentence, that's a break in the story. Too many distractions of that sort, and it's hard to stay involved.
It's the same with poor plotting and characterization. Every time I find myself thinking, "How did
that happen?" or "Why, by all that's holy, did he do
that?" I'm distracted from the story.
With good characterization, the characters can surprise you, but they never leave you dumbfounded. Even when they do the unexpected, it makes sense. Bad characters, on the other hand, tend to be puppets dancing to the writer's tune. Once you know what the writer has in mind for them (and with bad writing, a lot of things get telegraphed) they become predictable. They also, usually, become very, very stupid, walking right into every obvious trap the author sets for them, because it will advance the story. Characters who are stupid and predictable bore me, annoy me, and insult my intelligence. And when that happens I don't feel as though I am reading a
good story.