For me, a live surround mix can be discrete until the point where it takes me out of the "live" experience, then as a listener and mixing engineer, I tend to want to push more elements back towards the front again. But FX are key. The right amounts of reverb, delay, and compression are important for any mix, but they are especially key for live mixes, as the reverbs and delays help to give you the sense of being in a large space while compression can help to add a lot of needed energy to the mix.
That's why I'm especially looking forward to not only this live disc (when it arrives now in the middle of December for me) but also R40 (which should arrive any minute now!)
All the Rush Blu Rays released thus far sound atrocious.