Just about everything...ie:
Comparing it to the typixal 4x4 system you find in a pickup truck (like my F250) or expeditions and the like - You can't run them in dry weather in 4x4 mode.

They don't have the high tech differentials the VW uses so you need to "switch" from 4x2 mode to 4x4 mode.
Comparing it to most SUVs - they usually run in fwd mode until things get slippery, then a clutch feeds power to the rear...this is really awesome, if you start spinning out the clutch will engage about halfway through the spinout, giving you all the traction you need to blast yourself into a tree or ditch...
The VW system is always in AWD, has an infinitely variable center differential (from wide open to 99% lock), transmits power dynamically to the wheels with the MOST traction (the opposite of what most differentials do).
An easy test to see how superior it is is this: get opposite wheels on ground and the other two up in the air - ie driver front and passenger rear - you can do this in a ditch or similar. The Touareg will power out of this situation, almost nothing else on the road will since the diffs will just send all the power to the wheels with the least resistance...this is easy to visualize, how many times have you seen a car try and move on an icy surface, only 1 wheel on the axle spins - its the wheel with the least resistance. Only way out of the situation described (other than the VW or similar system) is a locking rear and locking front diff. which almost no vehicles have.