Theories abounded as to what was special about this instrument - ranging from the blend of woods to special treatments applied to the wood but essentially it seems to have been concluded that the key is a specific batch of wood that he used. Grown during a particularly cold spell in the 1650-1750's the wood was less dense than normal - but such wood is no longer available.
It should be mentioned that there has been great difficulty measuring much difference between a Stradivarius and more modern violins - even when panels of human experts are tried they often fail to identify the right instrument. Nevertheless they are still highly prized.
Insofar as I have been able to understand the low density of the wood allows the sound to travels through the wood easily, while at the same time its stiffness produced the tone. Modern woods are denser than those picked by Stradivarius but perhaps they could be lightened?
Tests of violins made with this wood failed to distinguish the new violin from the old (though we know that even with a non-treated violin that is far from easy). Enthusiasts claim that this may make the sound quality of a famous classical instrument accessible to far more people, and encourage young musicians by working with better instruments.