The Emperor is essentially the personification of evil. He's not only a tyrannical dictator who holds the majority of the civilized galaxy in his wrinkled old man grip but he's also a Grandmaster of the Dark Side of the Force and Darth Vader's fucking boss. Shit, I mean it's one thing that he seized power and overthrew a centuries-old republic, installing himself as supreme ruler of the universe, but throw in the fact that one of the most badass motherfuckers to ever live is completely and unquestioningly at his disposal and you've got a recipe for total badassery. Vader, Solo and The Emperor basically form the Holy Trinity of Star Wars Badassitude.
Oh yeah, and he shoots motherfucking lightning bolts out of his hands (!!) and probably even taught Vader that awesome Force Choke maneuver where you can crush peoples' tracheas just by looking at them menacingly and pointing. And while I generally discount the prequel trilogy as fan fiction and not actual canon, I have to admit that it was nice to see the fucking Über Dark Lord of the Sith totally whip the ass of CG Yoda and leave him for dead. And I'll bet you didn't think any Jedis could be more badass than Yoda. This is because you don't understand the true power of the Dark Side.
The Emperor also looks like the new Pope. It's the ultimate duality of man.
More than just being an unstoppable badass who was only defeated by being completely fucking jobbed to death by his own sidekick at the exact moment of his ultimate triumph, he's also a master manipulator and a diabolical power-hungry madman. The actor that played him in all five films he appears in (Ian McDiarmid) said that when he portrayed The Emperor, he was doing his best to channel Iago from Shakespeare's Othello:
"Everything he does is an act of pure hypocrisy, and that's interesting to play. I suppose it's rather like playing Iago. All the characters in the play - including Othello until the end - think that 'Honest Iago' is a decent guy doing his job, and he's quite liked. But at the same time there's a tremendous evil subconscious in operation. There's a moment in one scene of the new films where tears almost appear in his eye. These are crocodile tears, but for all those in the movie, and perhaps watching the movie itself, they'll see he is apparently moved - and of course, he is. He can just do it. He can, as it were, turn it on. And I suppose for him, it's also a bit of a turnon; the pure exercise of power is what he's all about. That's the only thing he's interested in and the only thing that can satisfy him, which makes him completely fascinating to play, because it is an evil soul. He is more evil than the devil. At least Satan fell; he has a history, and it's one of revenge."
Sure Bill Shakespeare might be turning in his grave at the knowledge that one of his characters has translated to a science fiction super-villian who builds space stations capable of destroying entire planets and kicks Ewoks, but if he didn't come to life as a zombie and start eating people's brains after Romeo + Juliet was released then he's probably going to be cool with this, especially considering that The Emperor has some of the best lines in all six films because he's such a sarcastic evil bastard.
In short, the Emperor kicks ass. Think back to the worst, most evil boss you've ever had at any job and then multiply that by infinity and that's what he's like. Because while it certainly takes a special kind of megalomaniac asshole to be a workaholic slave-driving retail store manager it takes the all-powerful living embodiment of pure evil to be able to order Darth Vader around without fear of being chopped in half or choked to death.
"So be it... Jedi."