I think some purists were really worked up though by the confrontation between Gandalf and the Witch-king at the Gate. Whereas in the movie the Witch-king seems to have the easy upper hand (both practical and symbolic from the breaking of the staff), I think in the books they were either about equal, or Gandalf (the White) was the stronger.
Quite right, except I feel that I must emphasize that Gandalf was infinitely stronger than the Witch-king. Gandalf, a reborn Maiar
sent back to Middle-earth - unveiled in all his power - and set opposite of Sauron himself. At the time of the Battle of the
Pelennor Fields, Gandalf was second in power only to the
Dark Lord. The Witch-king was a withered ghost of a
man from years and years ago. Hence Gandalf's utter annoyance at the Witch-king's silly assault upon him at the White Gate in the book. The only reason Gandalf didn't tear him up right then and there was the pressing situation with Faramir.
Seeing Elves at Helm's Deep irked me, and watching the Grey Host sweep across the Pelennor made my stomach turn over. But the movies aren't the books in three-dimensional form; they are the books according to Peter Jackson, just as the thousands of paintings of famous Lord of the Rings scenes are interpretations of their respective artists. Peter Jackson decided (rather intelligently) to put all the emphasis of the movies onto the Ring, with as few deviations inserted where needed to tell as much story as possible. Under that umbrella, the Dead can suddenly be at Minas Tirith (because you, average movie-goer, don't know about their original mission to stop the Corsairs from stealing all hope away from the battle). The Elves can show up at Helm's Deep (because you, the average movie-goer, don't know that Lothlorien is under attack as well). Saruman can die atop Orthanc... all is still well in the Shire (because Frodo decided to set out to destroy the Ring, not save the Shire).
See what I'm rambling about? The only parts of the movies that I don't care for are when the core story arcs surrounding the Ring deviate from the book. Take, for instance, the character of Faramir. Now, I completely understand why the writers gave Faramir the qualities he starts with, only disregarding the Ring after growing in strength, but it still bothers me. Faramir, to me, was always that calm sea in the torrential ocean. He was the opposite to Boromir, and it was refreshing to see someone so spiritually strong. And I didn't like Frodo going over the edge within
Mount Doom. Sure, it brought a tension to the scene that wouldn't have existed otherwise, but still... It's way too cliche for my tastes.
And, for the record, if the Nazgul had spotted the Ring in Osgiliath in the hands of a Hobbit at the end of The Two Towers, then Sauron would not have attacked Minas Tirith at all. He would not have assumed Pippin had the Ring when he caught him gazing into the Palantir. Sauron would have said, "No, you can't have it, because I saw a Hobbit with the Ring in Osgiliath like... two days ago." Sauron would have sent all 9 Nazgul to Osgiliath, followed soon after by every army and scout force he had on-hand. The proximity of Osgiliath to Mordor would have told Sauron that the Free Peoples were attempting the unthinkable - that they would seek to destroy the greatest power available to them (after all, if they had the Ring and wanted to keep it, they would have not brought it so perilously close to him). Frodo would have been caught in Ithilien and killed. Story end.