Never start a sentence with "So". No. No, never. "But" can be even worse, in immoderation. (I totally do this -gulp-!)
Good writing is easy to read aloud. There are all those things they teach you about writing - vary the length and complexity of your sentences, utilize assonance, sibillance and alliteration, etc - but all of those things can be intuitively understood by doing one thing: reading it aloud. Can you read without effort and running out of breath? Does it have a natural cadence? Does it feel smooth, or do certain words lurch?
Speaking as the narrator, I "feel" good writing flow before I "notice" it. Like when I have been reading without fault for minutes and realize, "Hey, this YA romance author has silky smooth writing!".
It's easier to describe what happens with bad writing, which is difficult to read aloud. Breath control becomes difficult ("longest sentence ever no human could possibly say in one go" gaaaaasp!). I falter. Certain words are just not meant to be next to each other, and take many tries to twist out of my tongue. Everything is lurching. My brain gets tired trying to interpret the intention of the punctuation a split second before I get there.
This is a situation where the narrator hopes to "elevate the manuscript", making it a better listen than it is a read. They try to inject flow where there is none, and to some extent this is possible. But it is one heck of an effort.
I have an idea that J.K. Rowling's flawlessly smooth writing (I've read all of H.P. aloud with zero fatigue) is that excellent because she wrote it for her kid! To be read aloud! I'm betting she read the drafts to her very first critical audience with a pencil in one hand to make edits on the fly. She was writing with the voice in her head narrating it.
Good writing makes the character. Characterization is what makes books memorable. As the narrator, often charged with giving characters different voices, I know the authenticity of characters exists in their dialogue. I can tell when a character has lived inside an author's head, and when they are faking it. Faking is bad.
Sure, I can slap a voice on someone, but it's like throwing a can of paint at them (you're blue, blue is the simpering soprano, and red is the gruff grumpy guy). They aren't really red, or blue. They've just been tagged. In good writing, the voice is already there, existing and waiting, and it comes almost immediately to me when the character starts to speak in the book (super neat, I don't even understand how sometimes). As organically, the other characters sound different from each other because they speak differently, on the page, before I "slap a voice" on them.
In poor writing, all the characters speak with the same vocabulary and style regardless of age, gender, personality. When characters are bantering without any he said, she said's, and I lose track of which is which, and have to go back and count lines? That's a bad sign. Characters should be distinguishable by the words they choose, just like real people, beyond the order in which they talk.