About the ghost thing: It's just another force trick. You gotta train on it and be pretty powerful. Of course, Qui-gon didn't know he'd had to learn it, he didn't expect to die so soon, since the Sith were supposed to be extinct. After his death, the council probably decided that it was best to have the Jedi train to become ghosts after their death. As we can see, only a few Jedi managed to master this technique before the Empire rose to power. Or they learned it afterwards, since they ahd all the time. Whatever.
I made most of that stuff up, but i'm pretty sure i read somewhere it was a special technique.
EDIT: woopsy, i see soemone already said this. But here's my theory on the thing.