Is there any way to apply open-ended keyword tags to songs in iTunes?
It currently is not possible to add tags in iTunes like you can in other iLife applications. The only reason I can think of why its not available is that most people would just group their music by genre, and that be enough. 

Other than using the comments field or the like, there are really not many other options. In some searches I did come across an app called Qloud, but it seems to no longer exist.