There's a ton to say on this point, but this is central: in the OT, the main character is Luke. Luke learns the force, struggles between good and evil, makes a commitment, a sacrifice, and reaches victory. Vader, Leia, the Emporer, and so forth are all steps in his journey. They are foils, obstacles, etc. Vader especially take the shape of Luke's challenge and his decision, just like in the cave scene with the vision of the duel. Vader embodies Luke's struggle. Vader is a vehicle of Luke's developemnt.
I expected the new trilogy to turn that completely on it's head, and that excited me to no end. Suddenly, Vader would be the main character. We would see his own journey, his fall, and his eventual redemption. Luke would now be a vehicle of Vader's development, and the final scene would be first and foremost about Vader's ultimate victory, rather than Luke's. (they will always both be there, but the focus changes.) I thought this complete overturn would be awesome. The story of Luke, which could sustain itself, would be set against a new backdrop and support an even greater story.
What I found was the story of Vader subjugated to the history of the empire, special effects lust, and political drama. Vader is woven in, but while it should be the center-piece, it's a subplot.
I actually LOVE Hayden in the role (you all can kill me now). I think he's awesome and perfect. But the script, the philosophy of his development, and the attention it receives drain the new trilogy of what I, personally, thought it could be.
(Speaking of all this, this always gives me chills. What if Luke, upon his father's redemption, had just stayed there with him in the throne room and held him until the Death Star blew up? (they still take off the mask and have the same conversation, they just don't get on the shuttle) Would have been a very different ending, but what it does to Luke's character...)