This post and comments may contain spoilers about the Death Knight starting quest line. I've tried to be oblique about it though. This also may change before release, but I deem it unlikely at this point.
In western RPGs, there is a tradition of allowing the player to make some choices, and changing the world in response to those choices. In particular, western RPGs love multiple endings that depend on your choices throughout the game. However, because players in an MMO share reality, we don't make choices on quest lines. Rather, we play through the storyline, and our actions are dictated by the quest designer. In some ways, the designer acts more like an author or movie director than a traditional game designer.
For the most part, this works out pretty well. MMO quests are not deeply intricate, and most of the time the best choice for the story is obvious, and that's what the quest has us do. We may feel some sympathy for Edwin Van Cleef and his treatment at the hands of the nobility of Stormwind, but it's pretty clear he's gone nuts and needs to be taken down. That's really the extent of any moral dilemma in an MMO.
But what happens when the designer/author makes our character take the wrong choice?
This situation comes up in the Death Knight starting quest line. Your character has to make a choice, and she makes the wrong one. And it's not just the wrong choice morally, it's the wrong choice for the story as a whole. As a death knight, you're sent to do other immoral actions, but those work to forward the story. This choice works against what Blizzard is trying to set up in the relationship between the Death Knight characters and the Lich King. I can see the story unfold if the character took the other path, and it is much, much stronger.
In most stories where a villain is in a position of authority over the main character, there comes a point where the villain orders the hero to do something unforgivable. This sets up the rest of the character's arc. If the main character refuses, the story becomes about becoming a hero. If the main character agrees, the story becomes about the character's fall from grace.
The quest I am talking about is a perfect set up for this choice. Only the Death Knight chooses to fall. And that choice seriously weakens the rest of the Death Knight storyline. I can kind of see why Blizzard went this way. The idea is that this is supposed to awaken some feeling of goodness inside the Death Knight. But that awakening is never demonstrated, and thus it feels like the awakening never actually happened. In a lot of ways, the final climactic scene--which, by the way, is spectacular--is rendered hollow and feels oddly inconsistent because of the choice made in this quest.
I'm sorry if this seems obscure. I'm trying to avoid spoilers. If you're interested, I'm talking about the quest A Special Surprise. Search for it on Wowhead, the quest description pops up if I link directly to it. Ordinarily, I'd just delay this post until after Wrath is launched, but I'm hoping that Blizzard changes the quest. I don't think they will, but I can hope.