Perhaps the most unsettling thing about Revenge of the Sith is the way it turns everybody uncool. Darth Vader isn't cool -- he's weak, petty, and manipulated. Padme isn't cool -- no more picking locks and smacking alien panthers down with a chain; she can't even find the strength to hold on to life so that she'll be there for her kids. The Jedi aren't cool -- they deliberately piss off Anakin and set him up for failure, even though they know there's a Sith Lord out there somewhere monkeying around with things. And Obi Wan Kenobi isn't cool -- he's bigoted against droids, perfectly willing to sacrifice clone troopers, too slow to recognize Anakin's vulnerability, and unable to recognize that the statement, "Only a Sith deals in absolutes," is itself an absolute.
When Obi Wan leaps up onto the bank of the lava river at the end of his duel with Anakin, he speaks two sentences that are very telling: "It's over, Anakin. I have the high ground."
Of course, we know it's not over -- there are still three movies to come. But at that point, Obi Wan has no idea that Yoda will fail to defeat the emperor -- no idea that the Republic has already fallen.
Likewise, Obi Wan only thinks that he has the high ground. Even as he admits that he's failed Anakin, even while he's telling him that he loved him like a brother, he is preparing to leave him to die a gruesome, horrifically painful death, alone and helpless -- and he makes sure to pick up the guy's light saber as he goes.
The prequel trilogy ends with evil triumphant because the good guys have dropped the ball -- and what's most interesting is that the pattern inverts itself in the original trilogy. Luke's triumph in Return of the Jedi is not a part of the victory over the Empire. It's a personal triumph for him, turning his father back from the Dark Side. But it does not effect the outcome of the Battle of Endor. The good guys get lucky that a bunch of furry Ewoks help them out, and that Han Solo pulls a trick on the shield generator crew that only an idiot would fall for. The Emperor would have died when the Death Star blew up whether Darth Vader threw him down the generator shaft or not -- and the reason he was doomed to die is the same reason the Jedi fell in Episode III. He didn't keep his eye on the ball.
The message of the saga as a whole ends up being pretty simple: politics will come and go, and those with power will sooner or later grow overconfident and assure their own downfall -- so be good to the people around you, because that's what will ultimately make a difference.