There's no shortage of critics of Dirk Nowitzki today, although TrueHoop has the best suggestion for the German star: refuse to accept the MVP award.
But this is not the 'biggest upset in NBA history.' Dallas won 67 games by healthy for most of the season (each of Nowitzki, Josh Howard, Jason Terry, DeSagana Diop, Erick Dampier, and Devin Harris played in 70+ games; Jerry Stackhouse played in 67), in a League that is injury-producing, and that has a regular season that is (a) too long and (b) means very little.
Golden State, on the other hand, survived a long season despite Baron Davis' knee surgery in February, and a major trade. (Most similar player to Davis, by the way (according to Basketball Reference), is former Knick Ray Williams.)
The Warriors appear to be peaking at the right time, are better than their record (42-40), and possess a clear home court edge. With neither the Rockets nor the Jazz in possession of a major low-post threat (a la Tim Duncan), there's no reason think the Nellie-ride will end soon.
Finally, on a personal note, as a three-time alum of the Nelson/Sanders Basketball Camp in Durham, NH, congratulations to Don Nelson.