iTunes music sharing provides extremely simple access to the music without any of the benefits normally found when using the full iTunes UI (browse and sort by album, etc.)

Apple TV may use the same iTunes sharing but may also create an index of the music locally (on the unit itself) as it seems to have much better browsing and control abilities than using a remote iTunes connection.

Running an instance of SqueezeCenter however, provides a fairly nice web interface to browse and search the music database and control playback on any number of Squeezebox units or software-based SoftSqueeze instances. With much better tag treatment and support that iTunes as well, which can let you make really fancy playlists or selections.

TV-based playback-wise, the Apple TV will probably kill off most of the competition sooner rather than later.