iTunes' star system of ratings isn't the best fit for classical music. I suggest that Apple add a new column to view options that uses symbols to rate the moods of a piece of music. I saw this sort of rating used in Jeremy Nicolas' book, "The Classical FM Guide to Good Music."
For each piece that he reviews he assigns one or more symbols: a trumpet for celebratory music, a heart for romantic music, a tear drop for sad music, a sun/cloud for relaxing music, a candle for inspirational music, and a champagne bottle for another category (I forget which). Similar symbol ratings could apply to popular music. Apple could offer a more extended set of symbols from which the user could select those that are most relevant to his iTunes collection. Let me know what you think.
dfbills adds: You could easily accomplish this with keywords, but it is an interesting concept