Regarding Lotr, I have only two real complaints. The elves at Helm's Deep served no literary purpose what so ever. They showed up, died a bit and disappeared. Were they ever mentioned afterwards? And I would say that it lowers the stakes a bit and ruins the tention somewhat. It was still pretty damn good so I am willing to overlook it.
The other is a bit worse. The Army of the Dead, *God Mode Engaged*, the whole host of evil crumbled beneath them and would simply not stand a fraction of a chance. In the books he arrived with the Grey Company, representing the Dunedain of the North, as well as the army of Dol Amroth representing the Dunedain of the South. Meaning that he arrives at the head of a army of Arnor and Gondor reunited as the last remnant of the royal lines. Hence "Return of The King". All the symbolism was lost in the movies, and we saw a total of what? 4 Dunedain?
The original version would be just a lot more powerful and emotional, giving them another scene of a massive host marching into battle. Hell, we could actually have seen some easterlings fight while we are at it.
OT: V for Vendetta, the original work was far from bad. It was a masterpiece, and really hit home. Yet the movie delivered it with a superior force thanks to the brilliant acting. I know none improvements to the movie that could be made, but I'm sure there is something somewhere.