Debate time!
Did Vader kill Obi-Wan? Or did he become one with the Force right before Vader's blade struck him?
I feel that Obi Wan surrendered himself body and soul to the Force before Vader's blade struck. If Vader's blade had struck, then there would be a body. Vader seemed surprised at the lack of a body using his foot to double check. This would have been a surprise to Vader as he most likely did not know anything about what Qui Gonn ability to communicate after death and at the time only Ben and Yoda would have known. It also would probably have been the first time (at least in recent Jedi history) a Jedi disappeared in this way.
I would also have to believe that in the case of Yoda and Luke, they utilized the last bit of their life to surrender to the Force before actual death.
I think over the years, especially for those of us who only knew the OT, we had made an assumption that Jedi just vanish when they die. Sort of like how vampires in the Buffy verse turn to ash when they die. Also back then, we did not know that Ben's ability to communicate with Luke was something that was rare. We assumed as a Jedi, when you die, it was a natural thing. However it would seem that in order to that, you would need some sort of training or skill prior... for the most part.
Qui Gonn is interesting... What was so special about him? I sort of feel he was at the time, the truest of Jedi, not the most powerful but the one who was the greatest follower in the Force and the Jedi Way and not entangled in the political agenda of the council which I believe was the primary factor of what caused the imbalance in the Force in the first place. I think he was strong enough that even after death, he was able to communicate with the living. Was it limited? Could he be seen as a ghost? I don't know.
Did Qui tap into a lost ability? Was it something that Jedi were able to do in the past but over generations became a lost ability due to imbalance? Also is this ability limited? In the old EU, Ben appeared at the beginning of the Thrawn Trilogy and spoke to Luke explaining that his communication would be his last as his abilities to appear to him were at an end. That would explain why you don't have 1000s of ancient Jedi ghosts wandering around.
Anakin's ghost appearance at the end of ROTJ is just odd (no matter what age he looks). He had no training or was probably even aware of the ability. I could only conclude that his appearance was assisted by Ben and/or Yoda as a one time thing to show Luke that he did succeed in returning Anakin from the dark side.
