In this September 14 post- which drew the third-most Comments of any post I made this year, I argued that since people tend to partake of music while in couples or groups, then Microsoft Zune's wireless-enabled, player-to-player music sharing feature would prove to be a killer app that iPod doesn't have.
Well the Zune was released with this feature intact- but Microsoft was attacked more than praised for all the restrictions this intra-Zune file-sharing offered. That, and the strong install base of the Pod, has not resulted in strong sales for Zune to this point.
I personally think part of this issue is the fact that Microsoft has not marketed the Zune's sharing features emphatically enough. Or maybe I'm wrong, and sharing is not as important from device to device as it is when live music is heard in a concert or club setting?