[Just your average movie goer]

Not only did they make Leia the other and cop out on introducing some exciting new character, they also completely threw the concept aside without exploring any of the plot potential there.

The idea of the other suggested that Luke would not be able to conquer Vader and the Emperor on his own or he would fail altogether.

However, seeing Leia's role in the movie, it really doesn't matter if she was the other or not.

Let's look at this line from Luke to Leia -

"If I don't make it back, you're the only hope for the alliance."

It sounds like the stuff of sweeping screen epics but it really falls on its face because the alliance would have defeated the Empire even if Luke hadn't confronted Vader and the Emperor. So whether Leia could take Luke's place was a mute issue.

Also, she takes her great force powers and does absolutely nothing with them during the movie.

Ah, what a waste. Return of the Jedi is one hell of a missed opportunity.