Well,
The music for one, and the moves themselves are just fun and inspiring. Each step/blast combo has just the right mix of variety and repetition to get the burn and keep your attention. It's tough, but something to work up to, and then when you "DO" it all the way through w/o modifications it is such a great feeling of flow and accomplishment.
Yes, there are a fair number of high impact moves, though they can be modified somewhat. There are airjack, tuck jumps, straddle-tap repeaters, power 15's, plyojacks, straddle hops along the board, and "genie jumps" (after completing a power box you hop on one leg approx. 16 times, then repeat the whole cycle again).
I'm sure you'll get at least a few other raves about IMAX2 as well as some specific suggestions for modification, you may even find a few old posts concerning this if you search back far enough.
HTH,
Mattea