My take on this subject is that Obi-Wan and Yoda both allowed their deaths to happen. They didn't fight to stay alive. Qui-Gon and Vader succumbed to battle death. Obi-Wan and Yoda both knew their time was up so they let it happen. Kenobi let Vader kill him and he disappeared. Yoda let the force in him give out, died and disappeared. Qui-Gon was killed by Maul in battle, died and didn't disappear. Vader's hand was cut off by Luke therefore setting in motion his death, and therefore did not disappear.

If Qui-Gon and Vader saw their futures and knew that they needed to die and were going to do so, I believe they too would have disappeared.

Of course George Lucas will probably dispell this theory with some strange idea but from those I have shared it with in the past it's a consensus pick on it being the closest to reality. ::shrug::