Whether you learn to write by reading thousands of books or through courses, there are some basics that should not be ignored (unless you're breaking the rule with style).
If you're writing a novel, do not write it like a movie script. These are two different styles. In a novel, there should be motion and emotion lacing through your dialogue. I love to write dialogue. I can quickly lay it down like train tracks through the setting of my plot, returning later to add scenery. But you should never strand two characters talking in one position just to relate information you want the reader to know. It's insulting and boring. (I will say, though, someone probably has succeeded in doing this, but they are the exception that proves the rule.)
|Exerpt from Thelma & Louise|
Don't use dialogue to quickly sum up all the pertinent information. Yes, this is mentioned above but it bears repeating. Dialogue needs to be natural. People do not just announce their intentions, their history, their expectations. Also, read your work aloud and if you stumble, edit. Frankly, I think it's an asset to an author if they like theater. Even if they never spoke a word on stage, if an author can voice act sitting alone at their computer, they will write tighter dialogue.
I often marvel at writers who use minimal dialogue. For me, it turns into telling, not showing. Like many, I am heavily influenced by films and TV. Books are definitely affected by writers growing up with the idiot box. But this is part of our culture so must be evident because all writers are influenced by their experiences. (Again, just be careful you don't write your novel like a film. It rarely works.)
If you're writing third person omniscient, don't slip into third-person multiple. Know your POV and maintain. I see this POV rule broken all the time. I had it hammered into me that you can't have the personal thoughts of two different characters in one scene and yet, I've read hundreds of books where this appears throughout the story. This is a case where the academic does not agree with the copious amounts of published work. Regardless, be careful of POV slips.
Without talking too much shop and rules, if the POV begins sweeping then suddenly turns intimate, then sweeping, then intimate with more than one character in less than a chapter, the tone is skewed. It's a delicate dance so don't leave your reader without a reliable dance partner guiding her through the intricacies of plot.
It is my preference to dedicate a chapter, or section of a chapter separated with * * * * * * to one POV. That way the reader has an indication that there's a break. This affords a smoother transition to the reader. I hate having to stop reading to figure out who is talking.
Those are a few points that instantly turn me on or off a book. These, more than spelling and commas, tell me that the book is still a rough draft.