WRAL has a really great feature up on DeMarcus Nelson and the season he's having. It also delves into how he got to this point - basically it comes down to the old Carnegie Hall joke - practice, practice, practice.
He also talks about being versatile enough to guard point guards or power forwards, which is something not many people can do.
Duke is widely perceived as being undersized, and it is true that they are shorter than maybe they'd like to be. But both Nelson and Gerald Henderson have serious hops, as the two pictures below (linked from GoDuke.com) show: that's B.C.'s 6-11 Tyrelle Blair, the ACC's leading shotblocker.
Clearly, height isn't everything.