I'm totally with you: the reason the prequels fail to interest me in the long run -- and why I won't probably watch them again, while I'll continue to see the originals over and over -- is that as you say, they did not really tell their own story, and did not alter the way we see the original story.

The only exception to this that I can think of is that the "Clone Wars" turned out to be a war the Republic fought using clones, instead of against them as I had assumed. That introduced an element of moral ambiguity (cue Yoda giving the command to send into battle the troops that will create an Empire and genocide the Jedi Order) which never went anywhere. At all.