I want you to think of the words "The Empire," not even the full and proper name for the bad guys of the original trilogy. It's sharp and evocative. When you think of those words you can see the Death Star, hear the Imperial March, and the clattering armor of a platoon of Stormtroopers. Darth Vader strolling through a fire fight, and his asthmatic breathing, looming over everyone else in his stark, black armor. The whine of a tie fighter as it flies off camera. An Imperial Star Destroyer dwarfing Princess Leia's transport after immobilizing it with a tractor beam. The Empire is monolithic, ubiquitous and a sharp contrast to the Rebel Alliance.
The waters in the prequel trilogy are considerably muddier. We have the Trade Federation, and seriously, who are they? They sound like a rotary group. Not exactly the stuff of nightmare. There's the Sith, who at least after we've seen Darth Maul seem a little more threatening, but you get very little back story. Sidious (Palpatine), Maul, Tyranus (Dooku) are the only Sith who are clearly identified, and none of them are very intimidating save Maul who doesn't last very long.
When we compare these villains to Darth Vader, they come up decidedly short. Vader was the stuff of our worst fears long before Lucas created him. Large and physically menacing, Vader was played by a Swedish wrestler. Sweden seems to have a history in producing science fiction and horror villains, Tor Johnson who was also a Swedish wrestler, was a staple villain in many older films. Vader was clad in black body armor, which provided both a mythic hook to hang the villain on from the countless legends of men in black and dehumanized him, as the armor obscured his features. His asthmatic breathing reminded us that he was a living being, not a mere machine. His evil is clear and unambiguous. He commits both murder and genocide by the time the credits roll in A New Hope. Despite the fact that his origins are purposefully obscured, Vader moves into his role as the principle villain of the original trilogy with little effort.
I understand that the evil presented by the future Emperor Palpatine is purposefully different. He's from the "evil mage" school of villainy. But I feel both trilogies and especially the prequel trilogy fail to present this well compared to more conventional fantasy. His evil in the prequel trilogy is subtle, but as in comic books, subtlety is not a device that works well in fantasy. In fantasy there is typically a very stark contrast between absolute good and evil. In this genre, evil even when it is not strong in the physical sense, must have the appearance of overwhelming strength of some kind - typically it should appear a great deal stronger than the heroes. Palpatine as a character fails in this, both in the final movie of the original trilogy (in which he is killed by Vader, Luke resists temptation and Anakin is redeemed), and in the prequel trilogy. We never see him as powerful, in fact, we usually see him as weak. This is communicated in both trilogies through his frail frame. Granted, he does best two Jedi and feigns weakness as part of his plan to bring Anakin Skywalker to the dark side, but overall he seems to be a one trick pony. Force lightning, Force lightning and failing that, yet more Force lightning.
We could compare heroes and see many of the same weaknesses. In fantasy however, villains tend to be more emblematic than the heroes. For instance, in Lord of the Rings, Sauron or even the One Ring itself is more representative of the story than any of the heroes. But I believe that the heroes in the original trilogy are stronger than the prequel in any case.
I believe almost anything Lucas had followed the original trilogy with would have disappointed. Similar to Joseph Heller spending his whole writing career living in the shadow of his most famous work, Lucas was too successful with the original trilogy and there was too much time between it and the prequel trilogy. By the time The Phantom Menace was released, the original trilogy was part of my generation's childhood, and changing anything in regards to those memories is to wander into dangerous territory.
