Sentence fluency and awareness of word choice are the biggest things i notice when i'm reading inexperienced/unedited prose tbh. And i'm not saying that because i thing grammar is immutable (although i do advocate for a good understanding of grammar so that you can break the rules intentionally rather than accidentally). It's just that bad sentence fluency is the easiest way to draw undue attention to your prose when the vast majority of the time your goal should be to become invisible under the story you're telling
I say this, of course, as someone who has recently been emulating the extremely flowery, attention grabbing prose of the 1800s, before the phenomenon of invisible prose was even really thought of let alone desirable. But sentence fluency is still important!! I guess what i mean is that sentence fluency helps the prose feel natural, regardless of the style in which you're writing, and imo studying/becoming aware of it in your own work is the fastest and easiest way to make your work sound more professional regardless of content
