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.