Too often, a spreadsheet has no errors, contains useful information, yet is a real pain to use. Why? Because the author did not give much thought to the appearance of the spreadsheet.
A well-written document is formatted for eye appeal - lots of white space, font size is not too small - and the same applies to spreadsheets.
If the spreadsheet is too cluttered, the user will be reluctant to even look at the spreadsheet, regardless of the quality of the information in the spreadsheet.
Pictures make data easier to understand. Use graphics for important information.
Use color sparingly.
Have as much white space as possible.
Simplify and summarize your important data. Don't make the data so complicated that the user has to decipher your spreadsheet.
Point people to the outcomes you want them to analyze. Users tend to focus on information in the top left corner of a page, so consider putting your most important data in that area.
Get a proofreader to identify the errors you missed and suggest design improvements.
Check the spelling!