One of Musk's original goals was to do it 10x cheaper, and accd to his recent discussion with NPR, he's already at 1/8th of the cost(1). What an amazing accomplishment.
1. http://www.npr.org/2012/05/18/153015277/on-eve-of-launch-spa...
1) SpaceX employs engineers who worked on the Shuttle, thus getting all of their lessons learned for free.
2) SpaceX gets to use facilities NASA built, like launchpads and testing facilities much more cheaply than if they had to build their own facilities. They tested their heat shields at NASA Ames, for example.
3) Everything is cheaper now. Not just raw materials, but imagine the cost savings when every engineer has a (Shuttle Era) supercomputer on their desk. Testing is dramatically less expensive.
4) Everything is better now. The (NASA developed) heat shields are much higher performance than those available in the Shuttle era. The level of computer control is much more sophisticated. Better technology means you can do more with less.
In short, SpaceX has massive, massive advantages over the original Shuttle team. I would hope that they could do more, more safely, at less cost.
But even if you want to grade on a curve, there's something to be said for appropriate use of what you have. It's easy to argue that the Space Shuttle was not a good design, even given 1970's technology, due to political constraints.
I suppose you could ask, "given both political and technical constraints that NASA was operating under, was it a good design?" but I'm not sure what the goal would be of that question. Is anyone blaming individual engineers for management problems?