Warning! Spoilers are contained!! ! Not that you haven't seen RotK yet...but I just thought I'd be fair and warn everyone. 
I assume we're talking about the movies here.
I've read all three books of the trilogy (several times each), The Hobbit and The Silmarillion.
The movies are well made, but I felt they cut out a lot from the third one. The scouring of the Shire (when the Hobbits traveled back to the Shire in the end, only to find that the Evils of Mordor had taken root in their Home) was my favourite part of the books and was a disappointment to be left out of the third movie. I liked some of the things they added in, but Arwen was around too much, IMO.
I was watching some of the book-to-movie adaptation stuff that they did (on the EE), and I'm just going to say this. If you read the literal, day-by-day appendix timeline in RotK, they line all the events up. The last two or so chapters of TTT were put into RotK because it fit within the timeline. Also, when we follow Aragorn, Gandalf and the rest of the Fellowship through TTT and RotK, their story takes up the whole first half of each book. Sam and Frodo take up the second half of TTT, but only the first four chapters of RotK. Because they cut out a few chapters near the end of the second half of RotK while making the movie (for example, the scouring of the Shire), it would make sense from a film maker's point of view to put a few chapters of TTT (featuring Sam, Frodo and Gollum) into RotK instead.
And, I'm sure PJ realised that it wouldn't be a good idea to leave Frodo almost dead at the end of TTT. Way too much suspense would be built for anyone to wait an entire year to see what happened afterwards. With the book, you can read TTT and then start RotK right afterwards (in theory, anyways).
Overall, good movies, better books. Epimonandas, just to let you know, each of the books is made up of two parts. FotR is made up of part one (Bilbo's party preparations to Frodo arriving in Rivendell) and part two (Frodo waking up in Rivendell and being reuinited with Gandalf and Sam to the breaking of the Fellowship). TTT contains part three (Aragorn discovers that Boromir is dead and Merry and Pippin are captured by orcs to Gandalf's meeting with Saruman) and part four (Sam and Frodo's journey starting when they leave the others, continuing into Mordor, meeting up with Gollum and ending with Frodo being "almost dead" and Sam being taken by the Enemy). RotK concludes the story with part five (starting with Pippin and Gandalf's arrival in Gondor and Aragorn's departure to the Paths of the Dead, following them through the siege of Gondor and ending with the Eagles coming) and part six (containing the rest of Sam's journey to find Frodo and their journey through Mordor, the Battle of Pelennor Fields, Aragorn's coronation and wedding, the Elves leaving Middle-earth and the scouring of the Shire, among other things!).
Sorry if I was rambling a bit there. I just thought I'd let you know that. 
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Itaque incipet.
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