In my mind his one mistake was a doosy. His fatal flaw was buying into Air Force propaganda that wars can be won solely by application of devastating strategic air power. There has always been a struggle between the AF and Army/Marine Corps concerning that particular theory, but starting with AF CoS Gen McPeak, the theory really began to be in vogue despite the objections of the ground services. Budgets reflected that buy in. It was very apparent, well before 9/11 that Rumsfeld bought into that theory. He had very little use for the advice of senior Army and Marine Corps officers who ended up being correct in their assessments. One more division (thank you Turkey) would likely have snuffed out the insurgency before it could start, since there would not have been a axis of escape and we could have controlled the Iraq/Syrian border.
Despite the miscalculation, Secretary Rumsfeld's ability to battle the insurgency and move toward success in Iraq and in the war on terror has been awesome. He is well respected by almost all who wear/have worn the uniform.
I think you've got it backwards. Rumsfeld didn't buy into AF propaganda, he simply believed that the two military operations he was ordered to orchestrate required less ground troops than popularly believed. In both cases, he was right; Afghanistan fell in a couple of weeks because we prioritized lending the Northern Alliance our air support, and in Iraq, well... you know the story there.
Regarding the terrorists fighting in Iraq now, that's more the fault of Paul Bremer's disbanding of the Iraqi Army than Rumsfeld's supposed lack of ground troops.
That's not a myth and is still true. The problem is that we never used "devastating strategic air power" in Iraq.