Rohan is the country that Theoden rules JM.
I agree with most of the assessment made above. Would like to add a few comments:
The Good:
The fight scence with the Oliphants was Great!!!! And I loved Gimli's comment of "It still only counts as one", continuing their contest from Helm's Deep.
The Scene where Aragorn commands Allegiance from the Dead was great, although I wondered about the absense of the other rangers esp the sons of Elrond who accompaied him.
The moments of Hobbitness exhibited by Merry and Pippin dancing in Edoras, and later after their return to the shire really came off well.
The Ride of the Rohan was incredible, the effects acting, etc. My only fault with this was that the army of Sauron seemed a little too prepared for them as they charged, I envisioned more of a panic and surprise, facilitated by the Pukel men. I also missed Theoden engaging with the Red Serpent who I had seen as a meeting of two Cavalry forces. Yet the Red Serpent design on the Oliphants made their substitution clear, and it was well done, yet I still longed to see Theoden best an enemy King in personal combat riding far in advance of his troops.
The scenes with Arwen that were not in the book at all were incredibly moving, a well done alteration here.
The Bad:
Denethor as stated above
No mention made of the relationship between Faramir and Eowyn. He is near death and she is heartbroken over Aragorn, then suddenly they are side by side healthy and smiling as Arwen and Aragorn are reunited.
The retreat of the rangers of Ilthein under Faramir even before the Ringwraiths actually appear, and he appearing shot up by arrows instead of a victim of the Black Breath of the Nazgul didn't make the final defeat of the Witch King as awe inspiring as it could have been. It also made the appearance of Aragorn as a healer and loremaster unneeded. An aspect of the King of Gondor that needed more work throughout all 3 films.
The time distance relationship near the end of the movie. When they keep flipping back and forth between Frodo and Sam headed up the mountain and the fight outside the gates of Mordor, it seemed to me that Sam and Frodo had far too much of a travel and climb to get there in time. Yet our noble hobbits seemed to make an ascent quick enough to make a comic book hero with super powers proud.
The arrival of the troops from other parts of Gondor was something else I missed, the mounting despair as each contigent is far smaller than expected. The absence of the Prince of Dol Amroth and Beregrond was also notable.
The exclusion of the power of the Voice of Sauroman, and Aragorns revealing himself, and Gandolf confronting the Witch King at the Gate, were also disappointing.
Worst of all perhaps was the defeat of Sauron at Pelenor Fields at the hands of the Host of the Dead, who never should have been there anyhow, taking away from the nobility of the defenders of Gondor and the Riders of Rohan. It cheapened what had been a most awesome epic battle until their arrival.
The Ugly
This is what I think could have been done to allow for many scenes that were left out due to time constraints.
Exclusion of the fight scene with the Worg riders in The two Towers, it wasn't in the books and really served no purpose. Elimination of Frodo's fight scene with Gollum after escaping the web and Shelob the first time. Elimination of the scene where Pippin is singing as Faramirs rangers are charging, replacing it with the brave rearguard action as they retreat as in the books. Also eliminate much of the fight scenes at the river from the Return of the King and the eariler one at Osgiliath in The Two Towers.
With a little better management I think at least some things could have been included that were lost.
Now I did enjoy the movie thouroughy, and I think Jackson did a good job, but I think it could have been even better. Hopefully an uncut version on DVD will include many things left out at the theather.