Right. I watched the Director's and Screenwriter's commentary on TTT tonight, so I'll comment on what they said on the changes.
On Éomer and Erkenbrand - Not really an issue, pretty logical. Having Erkenbrand rescue the rohirrim in Helm's Deep would have added another character, and been an unnecessary Deus Ex Machina, not to mention making Éomer a pretty much redundant character.
On The Cliff Scene - Firstly, Fran Walsh and Philippa Boyens make some jokes, then blame everything on Peter Jackson. PJ sais that because no chief character died in TTT, the story wasn't in a low-point enough for him. He wanted to create a feeling of desperation in later scenes, and making everyone think that Aragorn was dead was the way to do it. Also, he wanted Aragorn to witness the march of the Uruks and make him a more involved character, as so far in the film he had been pretty passive. The fall also served for Aragorn as a decision making moment - whether he actually wanted to continue defending the Rohirrim and accept his fate, or just... well, die. And lastly, if he hadn't fallen, it would have made the Warg Attack scene redundant. Which is a pretty pathetic explanation by itself, considering that the Warg Attack was never in the book and was something that PJ himself wanted to do.
On Faramir - It's been mentioned here before, PJ said that having Faramir reject the ring would have made him a terribly 2-dimensional character, as well as reduce the effect of the ring, as suddenly this guy comes, who the ring has no effect on, while for the rest 1 1/2 movies everyone's been yapping about how the ring corrupts everyone.
On Elves in Helm's Deep - PJ considers it just a cool thing to do, to increase the anticipation even, plus the other movies have established this kind of connection between elves and men (i.e. The Last Alliance), which he wanted to pay off. Also he sais that he wanted to do the part in which Elrond sends his sons to help Aragorn, but couldn't, for obvious reasons. So instead he had Elrond send Haldir.
On Entmoot - Basically, if the Ents had decided to go war in Entmoot, Merry and Pippin would have been basically luggage to be carried around in the first two films. But PJ wanted to have them make a difference, and expand their character arcs (Pippin's especially)
On The Osgiliath Flashback - PJ wanted to introduce the relationship of Faramir and Denethor, and show the entire family together. He also wanted to explain Boromir's motives. And explain the frase "A chance for Faramir, captain of Gondor to prove his quality". Logical enough.
On Leaving Out Shelob - PJ figured, that the dramatical ending of the film was the victory at Helm's Deep, and the emotional climax was Faramir letting Frodo go. So putting in Shelob on top of that, would have seemed like action for action's sake for the casual movie-goer. And cronologically, it happened during Return of the King, so that added to it.
On Stuff that Didn't Make the Film - Originally, as in back when LOTR was a two-movie project for Miramax, the decision to evacuate Helm's Deep was made because of a Warg Attack on Edoras, during the night. Aragorn would have then been dragged away by a rampaging warg and left for dead. This didn't make the cut, because it was impossible to shoot at Edoras during night, due to severe lighting issues. This ended up obviously as the Warg Attack during the evacuation
Also, Elrond was supposed to go to Lothlórien for council from Galadriel. Later, Arwen would have gone to Helm's Deep. This was cut because the screenwriters felt it didn't really go with Arwen's character and diverged from the book too much. This ended up as Haldir showing up at Helm's Deep.
In the scene where Faramir confronts Frodo, and Sam pleads him to let Frodo go, there is a moment when frodo goes to hide in the corner. This moment was originally supposed to be followed up by Faramir looking at Frodo and seeing him as a future Gollum mk. II, to illustrate what the ring was doing to him. This ended up being cut due to confused faces in New Line.
Discuss.