One thing I've seen far too often is wrong formatting in a manuscript. Most editors want the manuscript double spaced and an indent for the beginning of a paragraph, not a space between paragraphs. Before sending off to a publisher, check the guidelines and do what is asked for.
Active voice works so much better than passive. For an example:
Passive: Mary was strolling along the pier.
Active: Mary strolled along the pier.
When ever possible get rid of sentences with the was .....ing combinations.
Use strong verbs instead of adverbs to describe the verb.
Start a new paragraph when someone new does or says something.
Break up some of the long descriptions with action and/or dialogue.
Be sure something exciting or some conflict happens in each chapter—end a chapter with a reason to go on to the next chapter.
Use a person’s action or a description for a dialogue tag when possible rather than said, etc.
Remember, the point-of-view character can’t see how he or she looks.
Be sure that each character is unique and different.
Put color, smells, sounds into the description of settings.
Make sure everyone doesn't drink the same thing unless there is a reason for it.
When something exciting is happening, use shorter sentences.
When the dialogue is exclamatory, the reader can tell, you don’t need the exclamation points.
Stay in one person's point-of-view for scene. Should be the person who has the most at stake in that scene.
Only one space between sentences—not two.
These are just a few of the suggestions I've written on writers' judging forms.