One big thing I find when I proofread my own stuff is syntax and redundancy. I tend to write as if I was having a conversation with someone, and in a conversation that person gets to rely heavily on body language and tone of voice to understand you. Sometimes being redundant is a good idea to emphasize a point.
However, people don't get that in writing. They have enough time to reflect on what you've said and being redundant just makes it look like you're not aware of what you're writing.
I also tend to write as if having a conversation. I think that since I started posting on forums it is why I've always relied a lot on using emoticons to emphasise my tone - but they aren't something you can easily throw into a post or comment on Steemit, or even Discord. But I still try.