Yeah i think if someone ruined empire for me as a kid and told me who darth really was, i would have been pretty pissed off.

See when Lucas did this in film it was a cool and original idea. Before the Leia as a sister thing.

As a literary device it was old hat. As a mythic archetype it was already cliche.

Then there is the obvious even if you have not read the films spoilers before hand.

Like Watching Indiana Jones IV and instantly picking up that Indy is Mutt's dad even though the characters are oblivious.