|Please don't do this to me while I'm reading your story.|
1. Poor Grammar
2. Typos/Formatting Errors
3. Poor Writing (Unbeleiveable character actions, contrived plot line)
Good editing can fix all of these problems. I really think every manuscript should be edited by someone besides the author for content before being published, as well as copy and line editing. If you are traditionally published, your publisher should take care of all the editing. If you self-publish, you will have to find an editor on your own, but a good editor will only make your manuscript better. Don't let little mistakes keep your readers from enjoying your story. The best writing is a team effort.