As an author on here, I'm really hesitating to answer this question due the feeling I'll sound like an asshole standing on a gilded platform of my own creation, but okay...
DISCLAIMER: I don't know anything.
I've been trying to read every independent author here since the platform opened up. I've been digging through SiFi and Fantasy and frankly I've never gotten passed the first episode. Usually, the very first paragraph bothers me in some way and it warns me that I'll just continuously get bothered as I continue.
Spelling, grammar, punctuation, and structure are important. Of course. The story also has to have a proper flow of information without sounding like an instruction manual. But hey, I bet there's probably errors in my stories on here. I'm very poor at finding little things.
As an example, I tried to read some SiFi novel where the characters take a break from their travels as their mode of transportation takes a shit on them. But as I read it, I got the impression that it we were in a Mad Max world, which wasn't the case. I didn't get a proper description of the surroundings until we were already a couple paragraphs and many lines of dialogue in, and the characters were already walking around and doing things!
Also, one trope that eats me alive when I see it is repeated words. Again, I might be an offender. I try not to be. But last night I was like, "Hey, this one is good. This one is.......NO." There needs to be a variety to the vocabulary or it irritates the Hell out of me. Especially if you are referring to someting over and over, but only use one word to describe it.
Say we're outside and a stupid bird keeps flying in the yard and taking snacks meant for the squirrel. Because it's effecting the characters, it could be referred to as "annoying fowl" or "winged rat" or "feathered bastard" after an initial use of "bird." This one thing kept me from reading a story that actually started out pretty good, but gave me the impression the author had a very shallow word bank.
I think I only follow two independent authors right now and oddly enough, they are already accomplished comic creators. Their work isn't perfect by any means, but they are constructing a world and characters well enough and without any annoyances.
Please, take everything I say with a heavy dose of salt.