In 'Return of the Jedi,' the Emperor attempts to turn Luke to the Dark Side of the Force. For some reason, despite Luke's continuous whining through all three movies, the Emperor is of the opinion that getting Luke on his side would be worthwhile. So, he makes Luke as angry as possible, and apparently comes close to succeeding, but then somehow Luke's good side keeps the transformation from going through. Seems straightforward enough.

But what could possibly be going through Luke's head that would make this work? Here is the problem: how, by ticking someone off, can one possibly make them feel inclined to join sides with you? If someone told you that they were going to kill all your friends and that you would soon be their slave, you'd probably be a lot more inclined to tell them to bugger off (even if this involved some electric shocks) than to sign on with their club.

Which means that Luke's not turning to the Dark Side isn't much of an accomplishment - in fact, its no accomplishment at all. There's no triumph of the bond of sons to their fathers over the forces of evil, just some guy reacting the way most of us would if ticked off (to drive this home, imagine yourself in Luke's shoes).

In order to restore Luke's heroism, something needs to be added to the story, something a little more specific than 'the seductive power of the dark side of the force.' That is, if one tried to make the story plausible again by insisting that the Dark Side of the Force was surging through Luke, one would have to explain why, once he realized he had this power, Luke didn't just want to use it to go after the Emperor instead. When the Emperor says "Your hatred has made you powerful," why doesn't Luke, who one would think hates the Emperor more than anything, draw the conclusion that he'd be most powerful if he went after the Emperor?

Of course, one could insist on some cheap solution, such as, "beings like us, who have never experienced the Force, can have no sense of what was going through Luke's head." But that would make it all far less interesting - Yoda spends a long time talking about how the Force is really not much beyond what we're acquainted with: Life vs. Hatred and Death.

Here's another way of putting the problem: hatred tends to be directed at something. In Luke's case, the hatred is directed, at least in part, towards the Emperor. So the emotion that is supposedly supposed to make him join the Emperor has - as part of its content - something that precludes Luke having any desire to join.

Some hatred might be called directionless, when it takes the form of a general sort of loathing of everything (not uncommon among high-schoolers). One could also say that such hatred was the opposite of directionless, pointing in all directions at once. In either case, hatred of this sort would likewise preclude the joining desire.

By contrast, in 'Revenge of the Sith,' Anakin's turn is completely intelligible. He warms up to Palpatine because of the latter's flattery, then signs on with him in order to protect his family. Once he gets going as Vader, he gets carried away with the power the dark side of the force gives him. And once his wife dies, and the Jedi are taken care of, all he has left is his promise to Palpatine and his lust for power, so he sticks with it. This all makes sense. Here, Anakin's anger is never directed at Palpatine, so its not hard to understand how he would become bound to him.

PROPOSED SOLUTION #1
This is courtesy of Mike Raven. It's not a perfect fit, methinks, but it's the only non-hand-waving answer to the puzzle I've heard. According to Mike, what the Emperor is trying to do is to blow Luke's mental fuses. All the provocation is meant to do is to rattle Luke's mind, which is a way of breaking down his existing attitudes towards others: hatred, in the case of Palpatine, and schmatlzyness towards Leia and crew. That would definitely be a step in the desired direction, making Luke vastly easier to influence. Of course, exactly how that influence would work is still up in the air, but at least Palpatine's actions make sense.
This line of thought still needs some working out. For instance, if that's all the Emperor is aiming at, why do so by focusing on one of the attitudes that he wants to undermine? Couldn't he instead have thrown Luke in a mosh pit of Wookie strippers or something? But still - power to Mike for stooping to this level.