On topic, the ability to dissapear is possibly explained in The Unifying Force. Basically, for a Jedi to retain his consiousness after death, he must "fall into" the Unifying Force, which is like a step above "the Living Force" Quigon does this after he dies, so his body doesn't dissapear.
I was surprised to see in Attack of the Clones (novel) that Yoda didn't know that Jedi could retain themselves after death. He didn't understand this until Qui-gon started screaming for Anakin to stop killing the Sand people. Even Yoda didn't know everything about the Force.