As far as the new ‘introduction to Hobbits’ at the start…well, the background stuff between Frodo and Sam, and at the Green Dragon was very useful for the emotional buildup of the central characters, but, otherwise, I felt that the originally released version of the intro/Hobbiton part was more likely to grip the interest of an audience.
Further on, the Frodo/Sam relationship was deepened well in the additional traveling scenes.
Throughout the film, one could see where PJ had sliced a few seconds here and there to try to edit down to the 3 hour limit.
It was however, incredibly heartbreaking to see how much had been lost by the cuts at Lothlorien. The giftgiving for all was beautiful, and Gimli really grew as a terribly touching character.
The other thing that I think was tremendously amplified in emotional impact was Aragorn and Boromir’s relationship, and the importance of this relationship for setting Aragorn on his path to take up the challenge of his heritage. Yes, Boromir showed more terrible lust for the ring at Rivendell, held back only by the powers of Gandalf. But he also challenged Aragorn more to remember the world of Men and his heritage. “Yes, there is great weakness [in the world of Men]; but there is also great strength.” And at the same time the scene at the grave of Aragorn’s mother really underscored how distanced Isildur’s heir had become from Humanity — and thus from _himself_ and from confidence in his own strength to fight the Evil.
Boromir was seen killing many more Uruk-hai, and Aragorn had a heartbreaking added line before the tear fell from his face after Boromir’s death…recalling the exchange between the two at Lochlorien, when Boromir had dreamed of returning to the White City with him, and the guards calling out that the Lords of Gondor had returned. The guards would still call, but Boromir would not be there.
So in sum, the majority of the additions layered great depth to the emotional charge of the story for virtually all the protagonists, and these are the strengh of the Extended Edition.