the x2 wide low pressure off-road tires, x2 fenders and frames.
Swapping out the i2 fender and wheels for the x2 takes about 12 to 15 minutes. The x2 tires make the Segway too wide to fit thru normal doors and way too agressive for pavement and street use. But once off the beaten path they are just right for the task. However, there is a trade off, the wider (heavier ) tires take their toll on the batteries and limit the distance you can travel.
As a comparisson, from the official Segway site the i2 has a range of upto 38kms (depending on terrain) and the x2 has a range of upto 19kms! Of course, the i2 figure is based on pavements an streets and with x2, well, it really depends on where you go!