Richard Martin has a lengthy feature in Wired that clearly presents the case for thorium.
"It's abundant - the US has at least 175,000 tons of the stuff - and doesn't require costly processing. It is also extraordinarily efficient as a nuclear fuel. As it decays in a reactor core, its byproducts produce more neutrons per collision than conventional fuel. The more neutrons per collision, the more energy generated, the less total fuel consumed, and the less radioactive nastiness left behind."