One of the things that actually bothered me a bit was the Vibranium and how it was implied that it's the same stuff as Cap's shield. In the comics, I believe the shield is an accidental alloy of Adamantium and Vibranium, which makes it unique and obviously much more precious.
In AoU, it seems as if they're saying Cap's shield is only made of Vibranium (Ultron even says all humans did with Vibranium is make a frisbee) which makes it much less unique and important. It becomes replaceable.
If Ultron was covered in Vibrianium and still took damage then it strengthens the theory that Cap's shield is a special and unique mixture, which I welcome.