Playback Viewport
Playback Viewport reproduces what I think is a killer feature of XSI (well, for animators though) :
It automatically switches to a viewport designated by the user when animation playback starts, and switches back to the previously working viewport once the animation stops.
That might sounds a little weird, but they've designed a very comfortable workflow, especially when your viewport layout looks like : Facial Rig + Overall scene view + Final Camera view (the interesting one).
This way, you don't have to change viewports yourself (twice, before and after playback) each time you want to check your animation.
The character you can see on the screenshot is Dee, a great free rig by Manuel Sierra.