Cinisajoy's list hit many points spot on.
Spell check is NOT a cure all.
GIGO is still GIGO... even if it is spelled correctly.
Homonyms

is the big one that brakes (breaks

) my reading trance.
Technology errors is another one. Be sure to apply John W Campbell's rules: Right or explain it away. Get advice if it is not your field.
Your sample/teaser chapters need to be 100% clean. It is the old 1st impression rule. I need to look at your
story, not your writing. I am buying a Story.
Author requested reviews/likes... whatever inside the book: IMHO don't.
Maybe I am old school, but user reviews should be from the heart, not request. Enthusiasm of
MY fellow readers has sold more books than any fluff on a cover or NYT list.
My 2cents