I think there is confusion in the way that SOLO works now and in previous versions of RB. In 2012 there are solo buttons on each track whereas previously you could used the overall SOLO button and select which tracks to solo by highlighting the tracks togther with either the Shift or Control Buttons. The latter functionalty was an easy way to test groups of tracks and the highlighted tracks much easier to see than a host of small solo buttons The SOLO Button can still be used to cancel all soloed tracks.

I would prefer that the functionality between highlighting a track and being able to hear that track playing in Solo mode should be returned.