Generating functions seem old hat to those who have worked with them, but I think their early use could be another example. If you did not have that tool handy, could you create it?
Similarly, any technique that has been developed and is now widely used is made to look natural after years of refining and changing the collected perspective, but might it not have seemed quite revolutionary when first introduced? Perhaps the question should also be about such techniques.
Gerhard "Old Wheels Made New Again" Paseman, 2010.12.09