The new Drum Rack goes WAY beyond Impulse. It combines beat slicing and midi-automapping of slices (like Fruity Slicer or pHATmatik PRO) with kit building. It also supports REX files. It has a great system for supporting multiple grouping of pads (looks like in groups of 16). There is a vertical scroll-bar interface that allows you to "focus" on sets of pads. The set that is in focus automatically maps to your controller – awesome.
Ableton has posted a video to it’s web site that illustrates all this and a lot more. It’s not on YouTube yet, but if you go here http://www.ableton.com/live-7-whats-new, then scroll down the page till you get to the section Introducing the New Drum Rack.