this started as a comment to Niket's blog, but started becoming too big - so i figured i'd just post it here
i suspect the NDA lost because they failed to foresee the consequences of using italian made voting machines (i suppse that explains complaints that the machine smelled strongly of alfredo sauce!!)
but jokes apart, it has been an across the board rejection of the incumbent in all states which have come to prosperity in the new economy (andhra/karnataka in particular). i think its not those who prospered that made this rejection, but those who felt "left out"!
another theory i have is that the length of the election (3 weeks) acted against the government, esp after the early exit polls predicted losses for the incumbent. so the subsequent voters are unfairly influenced IMO.
and then there are other theories, nicely summarized by Amit Phansalkar :).