A poorly advertised new feature in iTunes 7.4 is the option to rate whole albums as distinct from individual items.
Rating in general can be tricky, as it’s arbitrary and one rating must necessarily be compared with another in order to be meaningful. Apple could have created 10 stars (and strangely enough with a bit of voodoo, you can rate with half stars) but that’s getting too complex.
Long ago I realised I could only effectively rate songs as they relate to each other on a particular album. It’s hard to compare The Dave Brubeck Quartet’s outstanding Take Five to AC/DC’s Heatseeker because they are so different, although I’ve rated both songs 5 stars. Therefore I have a lot of 5-star rated songs, but some albums I like more than others. This is where the album rating comes in. Spiderbait’s Grand Slam is one on the top of the list, but Bowie’s Heathen, although cherished, comes in a bit below in my personal estimation. Now I can rate them accordingly. This allows me to more effectively create playlists based on the best songs of all time. For example, I could filter 50 songs rated 5 stars but the album has to be rated 5 stars too. This guarantees the truly best songs. Conversely, you could create a playlist that filtered an album rating of 2 or 3 and cause yourself to reevaluate just how you feel about those albums.
Albums are rated automatically by iTunes, indicated by open stars. As soon as you rate one song, it starts averaging all the individual song ratings and applies this to the album. As you continue to rate songs, the average gets more accurate. You can override at any time and select a rating just like you do with individual tracks, by clicking a star, only you do this in Album View underneath the album artwork. This will change the open stars to closed stars, and iTunes will stop averaging the album rating. If you haven’t rated some songs and you define an album rating, it will update all the unrated songs with the same rating, also indicated by open stars, which can also be changed manually.
We can assume that you bought your albums because you liked the music, so it’s also reasonable to assume that there will be many highly rated songs that saturate the top two ratings. This made it hard to identify the truly great songs without this additional rating option.