Gandalf Vs. The Witch King

[Essex]

I think the reasons behind the changes to this scene were to do with Jackson's (mildly annoying) continuning reference to the 'world of men'. The WK mentions that it's the end of Men in this scene doesn't he? Gandalf is on the floor and I admit he looks frightened (but who wouldn't be?) but not YET beat, waiting for the WK to land a blow. Then the Horns of the Rohirrim blow, and the "World of Men" save Gandalf (and assist in finishing off the WK later)

RE the WK leaving at once. As the book states:

But it was no orc-chieftain or brigand that led the assault upon Gondor. The darkness was breaking too soon, before the date that his Master had set for it: fortune had betrayed him for the moment, and the world had turned against him; victory was slipping from his grasp even as he stretched out his hand to seize it. But his arm was long. He was still in command, wielding great powers. King, Ringwraith, Lord of the Nazgûl, he had many weapons. He left the Gate and vanished.

This is why he left. He didn't have a second to spare and could get back to the Wizard later no doubt.......

PS I think we see Gandalf's staff being broken to show the power of the WK, and this heightens Merry's and Eowyn's bravery and their Deed a little later on in the film.