I think this is actually a major reason that in the prequels the Jedi talk about a set of "trials" that have to be faced before your Jedi-hood is official. Training alone isn't enough for Jedi status. I think that was put in the prequels to offer the explanation that Return of the Jedi was lacking for why Luke had to confront Vader. Since the Jedi have been disbanded in Luke's time, the official trials that existed in the Old Republic days aren't available to him anymore, so his circumstances have to be used to create a custom set of trials for him.

Mind you, I'm not saying this explains away what you guys have said about Return of the Jedi, since I hate the prequels and don't think that the original movies should require the prequels to make sense. But two things I would say in defense of Return of the Jedi are: 1) everything that requires training also requires testing before official "graduation," I can't think of any examples from the real world that contradict that, so we don't need the prequels to justify the concept; and 2) when a movie leaves room for the viewer's imagination to fill in certain details that are not fully explained, that doesn't always have to be seen as a flaw (though it sometimes is)