I've been hearing from Correspondents that the half-stars they had meticulously set in their track ratings have disappeared. I think one of the v10 updates wiped out the half-star preference or something. Anyway, you can bring back the half-stars by doing the Terminal thing (quit iTunes first):
defaults write com.apple.iTunes allow-half-stars -bool YES
Or you can use Change Hidden iTunes Preferences which has an option to toggle 'em.