But folks have been pretty good so far about spoiling it! Answering this question would entail a chronicle of Jackson's changes and inventions. He could at least save the invention for the DVD.
I guess I wonder that Jackson was able to carry us well into The Two Towers in the first film, but he couldn't get us out with this one. It would've been a good exit, for example, if the end of TTT would've had Aragorn entering the Paths of the Dead, and Frodo and Sam entering the tunnels beneath Cirith Ungol.
The second half of Jackson's TTT holds the bulk of the divergent material. Many of the changes are meant, seemingly, to develop the characters. They are meant to instill doubt, restore hope, or underscore the motivations of key characters and at critical times. Fans of Tolkien and general movie-goers with no interest or knowledge of Tolkien accepted these same characters as presented in FOTR. Why the need to play so radically with the sequence of events from the books to round out characters we already accept? The first half of the film was more akin to the pacing and mild(er?) changes that we saw in FOTR.
It's a good movie. The story is good, the acting uniformly solid, and the effects superb. Were this an orignal Jackson movie under a different name, we'd still be talking about him as a revolutionary in epic filmaking. That epic at this point in time, IMO, is not now Tolkien's The Lord of the Rings.