The Zune player should be able to go forward or backward inside a song to allow a musician/user to listen several times the same part.
As the smartphone is becoming THE music player, this feature is a must have for any musician or user who want to study the music. It seems that only the Deezer player can do it.
On the Zune version of the PC, it is possible using the moving bar but the size is too short to use it properly. VLC do it very well allowing even to slow down/up the tempo (a good to have).
You can do this already using the back/forward buttons. Just hold them to fast forward / rewind.