There is no doubt that the thing that most separates "good" writers from "bad" ones is consciously thinking about who is reading what they are writing.
Yes, there are rules about grammar, punctuation and spelling that should be followed, but those things have been safely ignored by others who have still found success in writing. Good writers understand how to describe things in such a way as to tap into experiences and feelings we all know, they are aware what people will know or not know and they write accordingly.
Bad writing tends to go off the rails when someone gives little to no thought about who is reading it. A common example I would cite would be in office email, where there is always at least one person (sometimes more) who is terribly ineffective in communicating via written form. These are people who use complex terms or acronyms without the slightest inkling as to whether the reader knows those things. They skip over large gaps of supposed knowledge that the reader might have, or they talk down to them, explaining things in minute detail when it is not necessary.
Bear in mind that this writing often has all the hallmarks of what a non-writer might consider "good": perfect grammar, excellent punctuation, perfect spelling and deft use of adjectives and adverbs in descriptions. But it's not good writing because it did not successfully convey the writer's intention to the reader.
On the flip side, you might see a text from a friend that says: "Its too cold." You first notice the missing apostrophe on "it's". But the meaning of the message isn't lost on you. You know what it's like to be cold, you have a clear picture in your head of what "too cold" looks and feels like. Even with the mistake, it's better writing than the colleague who asks" "Did you happen to see the detailed RQE from corporate?" when you have no idea what the RQE is.
Even when the second writer provides context, such as adding that the contents of the RQE means no one will be getting bonuses this quarter (and you deduce that it might mean "Reported Quarterly Earnings"), you're left to trip over it first then figure out the answer. The second writer didn't understand who was reading their writing, and made an assumption that you knew as much as they did.
I always found it to be a useful exercise to actually imagine someone you know reading what you wrote. When I worked for the newspaper as a reporter, I envisioned my grandparents reading my stories and wrote them in a way that matched their knowledge, vocabulary and need for details. My stories were generally well-received, and I was considered a "good" writer. Taking a few extra moments to imagine an audience usually does mean the difference between writing that's hailed as good versus writing that elicits no opinion or a negative one.
The genesis of the dreaded rejection letter from a publisher or publication tends to be rooted in audience as well. Too many writers make the mistake of not understanding the audience of the publication. The writing may be excellent, but not suited for the publication. The rest of the puzzle, figuring out the whims of the editor who chooses the pieces for publication, is an exercise in targeting an audience too, albeit one that is far more difficult.
It might sound silly, but there is no harm in imagining all the details of the editor as you write and prepare your submission. You should think about what he or she had for breakfast. What is the trip to work like each morning for that editor? Traffic jams? Missed trains? Rude fellow commuters?
You get the idea. The point is that you would probably write differently knowing exactly who is going to read what you've written. There are lots of grumpy people with lousy commutes that drink instant coffee. They're going to like your story. They will hate it if your story is about someone who wakes up cheery, goes straight to the counter at the coffee shop and orders some fancy coffee that takes a barista 15 minutes to make; then hops in their car and drives for 10 minutes into their downtown office where there is ample parking.
If your goal is to write for a large audience, you're going to have to think about the characteristics those people share as part of your audience research. Your plot needs to elicit emotions that people can relate to, and the vocabulary you use needs to match what your readers will easily understand. It might sound like stupid advice or common sense, but ignoring audience happens all day, every day for writers around the world.
For any writer that wants to share with the world has to focus on who will be reading what they write, focusing on audience is the biggest step toward becoming an awesome writer.