I was with some friends the other day when someone said, "If Star Wars technology has lightsabers and blasters, why can't they just reattach Anakin's arm after Dooku cuts it off?"
The answer is actually fairly simple. Energy emitted from lightsabers is strong particle beams. When moved through flesh it severs and burns all nerves and blood vessels. This why you never see blood. This is also why Anakin's arm couldn't be reattached. All nerves, veins, everything had been burned. Yes, they could have put it back on, but he would have no control over it. Putting a mechanical arm was much easier and gave Anakin control over his arm.
As with Luke's hand, well, they just couldn't find that.(HA,HA)