Originally posted by: CO

The cave is such a great part of the movie because it is never explained to the viewer, it lets the viewer come to its own conclusion. Only Yoda says, "Remember your failure at the cave."

Yeah, the lack of explanation was great. Though, I still think its pretty clear that his failure was how he used the force to attack and kill the illusionary Vader (his striking blow was dealt with lightning-fast speed as I remember). Directly attacking with the force is of the dark side and Luke saw himself becoming like Vader, taking the easy path to victory. Well, even if nobody agrees with me, those are my thoughts.