I'm finishing a rewrite on a book that I have been picking at for basically forever and I'm doing edits on another with Canda. The thought that keeps coming over and over to us is sharpening sentences.
1- Getting rid of all sentences repeating what was already said or done. No echo's needed. :)
2- Strengthening the sentence with descriptive language to provide emotion/tension to a scene and paint pictures in the readers mind. Using more similes, metaphors, and all together lyrical language. This provides a distinction from amateur writing to professional writing.
This is what polishing is for us. Once we have the characters, plot holes covered and questions answered we look at sentences and how we can make them richer, giving the reader a more complex story to enjoy.