I have several problems with this. First of all, Qui-Gon apparently came back as a Force ghost to train Yoda and Obi-Wan on how to perform the same "trick." Well, problem #1 is Qui-Gon never disappeared - so how the heck did he become a Force ghost in the first place? Problem #2 is that if a ghost can provide Jedi training, why did Luke have to go to Dagobah when he could have just completed his training with Obi-Wan?
So this leads to problem #3. No, not why is Anakin's ghost at the end of ROTJ now a smirking kid. (I've given up trying to figure that out.) Why is Anakin appearing as a ghost at all? He didn't learn this trick from Qui-Gon. In fact, he acted kind of surprised after he killed Obi-Wan, so he obviously had no knowledge that Jedis could disappear like this.
Can anyone provide a reasonable explanation that somehow saves the greatest moment in ROTJ from now becoming a great big head-scratching moment?