These are the top ten right now because they're all things I encountered in my last edit!
Put your name in your MS. Spell it right. Seriously
NO FORMATTING that the pub didn't ask for. No weird style sheets, macros, highlighting, etc. Just no.
Run spell check judiciously. Don't accept all the things it tells you to do. Check for extra spaces between words. Track changes can do that
Put your website and social media links in your bio! There's nothing worse than finding a new author you love in an anthology and having to dig to China to find their other books, or to see if they have any. Most readers will give up.
Read your blurb. Out loud. Make sure it makes sense, tells enough of the story to hook readers but not tell the whole story, and make sure it's free of errors.
Do a find for em dashes and ellipses. Eliminate half of both.
Try to work with the publisher you're working with at their pace. They all do things differently. Don't tell publisher X that publisher Y is better. Even if it's true. ;)
Don't use chapter headings that don't say "Chapter" in them. In the day of digital tagging at places ibooks etc, even cute and clever chapter headings like, "Dog's day in heaven" should be prefaced with Chapter Thirteen. That's how they get tagged in ebooks.
Do your research. Put your states in the right place. Make sure you CAN actually use that as lube.
Everyone has a different voice, so stick to your guns, but make sure you're defending your voice when you refuse an edit, and not just being defensive. This is the hardest one for me as a writer, and I'm finally learning to let go.
I'm sure there are scads more but that's enough for now. What have y'all learned during the editing process? From either side.