I must credit the actor who played Denethor for the coolest line in the whole trilogy: In this extended cut, when he learns that the first levels of the city are in flames and that the men are deserting them, he says something like 'Why do the fools fly? They should stand and burn, for burn they must.' Awesome. Oh, and he has a heated exchange of words with his son that stand out because it's very dramatic. It even reintroduces Sean Bean as Borimir in a most unexpected way!
The House of Healing is back in as well, which is good, because it allows you to breath a little between the two big battles of the movie. Aragorn shows his healing skills, which allow us to realize that he IS really fitted to be king, and not just a wannabe. This new segment also has tender moments between Faramir and Eowyn while they're recovering from their battle wounds, which serves a nice conclusion for both characters and allow the actors who play them to shine.
Of course you must have heard by now that Saruman has been reinserted into the movie, but let me tell you, you're in for a treat! The sequence is over five minutes in lenght but I would have taken more...Saruman tries to rally Gandalf once again to his cause, but Gandalf doesn't fall in the trap. Also, Gandalf finally gets the chance to establish himself as the ruling wizard of Middle earth, when he says something like 'I am not Gandalf the Grey whom you betrayed, I am Gandalf the White and I cast you out of the order.' From this point the sequence takes a dramatic turn and you're on the edge of your seat until Saruman meets his fate. A sticky fate that turns him to dust!
A new character is introduced in the extended version: the Mouth of Sauron. He comes out of the black gate riding a huge black horse and basically he speaks for his master (who is after all a huge flaming eye!). The Mouth taunts the members of the fellowship and makes them believe Frodo has died after endless torture. He has particularly gruesome words for Aragorn.