This probably only applies to the Wii version, but the quality of the audio of the songs kind of sucks. The songs aren't album quality... it isn't as low as concert-quality, but it's like they all got together in a room to take a shot at re-recording songs in one take "the best they could" so they wouldn't have royalty conflicts with the album versions, or something. There's just something slightly off about it. For comparison, the songs from Guitar Hero III and World Tour all mostly sound fine, and high quality, so it isn't just my audio setup. Some have the same "artificial" flavor, so maybe the same phenomenon applies to those particular songs.
Anyway, the wife plays it all the time, but I can tell and it bugs me.