ChainsawAsh said:

The idea I've heard is that, since he "ceased to be Anakin Skywalker" when he turned to the Dark Side in ROTS, the Force ghost should depict him as he was before he turned.

I hate that line of reasoning because it is wrong.

Obi-Wan says "he ceased to be Anakin Skywalker." Vader says, "That name no longer has any meaning to me." They both think Vader is an irredeemable being, no longer Anakin. They are both wrong.

There's this other guy, Luke, the hero of the movie, who points out that it's Vader's "true name" and that there is still good in him and that Vader really is Anakin Skywalker. In the end LUKE IS RIGHT. Everyone else is wrong.