Given all of the differences between Python and C, I don't think it's even debatable whether or not the indentation rules in Python are the key difference. I'd say that typing and interpreted vs compiled are both so much more important.
But i have to admit that most python programs are a joy to read ( thanks to guido's insistence on indentation as part of the specs ) whereas most real-world C code is just horrible ( even code i had written a few years back )