When Genius Doesn't Quite Make the Right Choices
Although iTunes generally does a good job of selecting compatible music when you create a Genius playlist, it isn't perfect. In many cases, songs may have similarly tagged information, even though they are in completely different genres or styles.
In some ideal cases, this can be a good thing because it will combine songs that sound good together but that you might not ever think to play back to back with each other.
In other cases, however, the results can be less than desirable; for example, mixing a Coldplay track with a classical piano piece. (Classical music, by the way, seems to be one of the most common forms of music to trip up Genius.)
In other cases, songs that you might instinctively put together in a playlist might never show up in one that is created by Genius.
Although Genius isn't always that smart about picking good matches for songs, it does seem to be getting better over time.
This is because Apple didn't design Genius as a static solution. In fact, the feature is designed to anonymously combine analysis of your personal music choices with that of all other iTunes users who have opted into the Genius feature.
As a result, the algorithms used to generate Genius results have continued (and no doubt will continue) to become more accurate.
This is also the method by which Genius can suggest music to you based on your currently selected song (in addition to some good old-fashioned marketing techniques such as suggesting other tracks from a particular artist or album).
In some cases, when you select a song and click the Genius button, you might get an error message stating that Genius isn't available for that particular song. This is most common if the track you've selected is one that you either imported from a CD on your own or that came from a source other than iTunes.
That isn't to say that music not purchased from the iTunes Store doesn't work with Genius (roughly half of my iTunes library is from other sources, and the majority of it works with Genius just fine). This does mean, however, that either there is no available information in the Genius database for that song or the song is not tagged properly in iTunes.