Maybe I'm the last person to find out about this...but Word includes a tool that can help you to determine how easy people will find it to read your prose. To access it, go to the "tools" section of the menu, click on "spelling," then click on "options" and check the box that says "show readability statistics."
When the program has finished checking your spelling, a screen will pop up that gives you three statistics. One is the percentage of passive sentences (e.g., "the test was taken by ten people" rather than "ten people took the test"). Another is the Flesch Reading Ease percentage, which tells you what percentage of people who can read at all would understand what you've written. And the final one is the Flesch-Kincaid Grade Level, which shows which U.S. grade level someone would have to have attained in order to understand what you've written. If you're a non-U.S. reader, let me point out this is based on 12 grades, with the 12th grade being completing high school, usually at the age of 18.
To give you an example, I've just run this post through the program, and it shows that passive sentences make up 11% of the post, 57.3% of all readers would understand it, and the grade level required is 12.
Obviously these kinds of statistics are not so helpful for fiction writers, but if you're writing for newspapers, magazines, or for kids, or instructional materials that have to be easily understood, this could be a valuable resource.
