Sorry I missed this. From the start you get to pick from Bronze or Silver difficulty levels. Gold, Platinum, and Niobium (an element from the game) are unlocked after beating the previous, and just like in Diablo/TQ you keep your gear when you restart at the next highest difficulty.
As for number of hours, yeah 20-30 for the main quest if ignore most of the sidequests, but that's like saying Oblivion can be beat in 8 hours or some such. I can easily see someone putting in hundreds of hours, easily. Some people on the official boards have done well over 300 hours. The world is so massive, and the sidequests so numerous, that, well it's the game that keeps on giving.
360 GT = Diseasedyak
I'll be getting the 360 version for sure, feel free to add me to whomever wants to.