Came across this interesting tip that shows how to enable and disable half star rating in iTunes on Mac. Digging further, you can actually enable the same thing on Windows as well. And here is how, if you are interested.
1. Go to %appdata%\Apple Computer\iTunes folder.
2. Open iTunesPrefs.xml in notepad or other text editor.
3. Scroll near to the bottom of the file and add the following lines above the last second </dict> tag.
4. Save and close the file.
The half star rating will be enabled next time when you open iTunes. To make the half-star rating, simply double click the left edge of the star. Note that it’s not available through the right click context menu.
Enjoy, if you are the fan of iTunes.