Covering a lot of ground here.
My biggest disappointment, and the one thing that was missing from all three movies that was absolutely pivotal: that Sauron wouldn’t expect them to try to destroy the ring. I guess you can argue it was implicit, but … it seemed to me that without making that explicit, viewers who haven’t read the books won’t understand.
And I agree, the bit where Frodo shows the ring to the Nazgul in Osgiliath (in TTT) was dreadful. If Sauron knows that the ring is heading into Mordor, he guesses that they want to destroy it, and Frodo is captured pretty damn fast.
I think Sauron in the books comes across as much more menacing, because we see how he subverts and lures, both in Saruman and Denethor. Great lines from the book, like "Treachery is our greatest enemy’ (I’m going from memory, forgive me if I blew it somewhat) don’t have their counterpart here. Sauron seems only to be a source of armies and more armies, and the despair of Denethor or the plotting of Saruman aren’t directly attributed to Sauron.
I agree with everyone, Denethor is far more interesting as a tragic figure, deceived by Sauron, then as a simple lunatic, driven mad by the death of his much-favored son (one presumes.)
I find it awful that Aragorn kills the Mouth of Sauron, that’s no way to treat an emissary. I did like PJ’s interpretaton of the Mouth – very cool. There were a number of other bits where I liked PJ’s interpretation, like the White Hand insignia in FotR.
Having said all that, I like the movies fine. I’ll never see a better adaptation, no one is going to remake them in my lifetime, and he got so much right and on target, that my quibbles are small. 98% happy with them is a fair statement.