I agree with Wambat.
The dramaturgic changes are ok to a degree. Elves in Helm's Deep, well it's stupid and impossible (due to the huuuuge distance) but ok, granted. It's ok they left Imrahil and the Knights of Dol Amroth. It's ok Arwens character is expanded.
What upsets me, are the changes in character.
Above Glorfindel is mentioned. The scene at the Ford is much more powerful in the books: Frodo flees alone and, though already failing in power, he still despises the Ringwraiths crying: "By Elbereth and Lúthien the Fair, you shall neither the Ring nor me!"
I repeat myself saying: What a powerful scene! This small hobbit, wounded to death despises the most terrible enemies he ever met! The most terrible enemies there are in the world, apart from the Balrog and the One in the Dark Tower. But noooooo, PJ had to have this stupid She-Elf and Frodo is already in this scene the snivelling wimp that he remains until the trilogy's end.
Also Aragorn is such a laughing stock! Remember that stern, hard, proud man in the books. Really uncanny in the beginning and dangerous. And so proud it's almost uppish. He, the dispossesed stands in front of Meduseld and is unwilling to bow before the law of Théoden King to leave Andúril at the Gate, claiming his lineage proudly.
There's never a doubt that he is heading right for the throne of Gondor AND Arnor, that he indeed is the heir of Isildur and Elendil.
And last but not least this terrible scenes where he is reluctant to release the Wraiths from their curse (he's a heir of Kings, he had given his word!) and then slaying an EMISSARY!!! I almost vomitted. How hilarious, a tyrant would do this upon hearing bad news.
Film-Aragorn is no better than a dirty Orc-chieftain and barely better than the king of the oath breakers.
Another terrible thing is the uttermost stupidity of Men. War is brooding since quite some time in Rohan and Gondor. Erkenbrand, a lord of Rohan, had fortified Helm's Deep and repaired the stonework in the books.
When in the film the gate is broken I wondered: "Huh, that's supposed to be a castle gate? My grandpa's barn gate is more massive!" Also did you ever recognize that Helm's Deep does have a second wall, but in this wall there's not gate? That's so insanely stupid, it almost hurts.
Or this scene in the siege of Minas Tirith. The gate is broken and Gandalf cries, that women and children should be brought in the second ring. WHAT? You didn't do this before?! Hey, there are huge boulders being hurled through the air, buildings collapse and you didn't bring the old and weak to the TOPMOST! level? In the books, they had evacuated!!! the city long before the assault!

In the SEE there's also a great scene. The Orcs cross the Anduin at Osgiliath and one of the Gondorian soldiers sees them. He leans out and watches and watches and watches, and leans further out aaaaaaaaand is shot! D'oh! There you see evolution at work.
Treebeard's stupidity. "Oh my god, Saruman hew down the trees! You bastards!" Well, I could've told you! That's again so dumb.
Also the dramaturgization is sometimes way too extreme. Orcs, even Uruk-hai, are in general weaker than men. Aragorn never went with 500 men to the Black Gate (it were roughly 6,000) and there hadn't been a chance to defend the Hornburg with 300 men (in the books it were about 2000 IIRC). Still those 2000 had to fight 10,000 Uruks. 2000 or 300, does it make a difference in this case? No, but it wouldn't seem as hilarious. Also the Gondorians contributed to the Battle of the Pelennor and joined the charging Rohirrim (in the films they are really cowards) and number of Mûmakil is plainly stupid.
Also some 'crookings' of the storyline are simply superfluos. Saruman making a snowstorm? Heck, why? Saruman inventing Uruks. Nay, he didn't invent them. The Uruks chasing the Fellowship from Lórien down to Rauros? Lol, wanna see 'em runnin'! The Army of the Dead liberating Minas Tirith? It's the same as with Frodo at the Ford: I wanted to see the Free People winning this battle on their own! With their own power they had.
Buhuhuuuu, Arwen's gonna leave, and than the "Surprise, surprise!" wedding! Gotta vomit.