I wasn't too fond of the Host of the Dead showing up at the Battle of the
Pelennor Fields. It sort of cheapened the sacrifices made by everybody else, and I'm sure some moviegoers who didn't read the book were wondering why Rohan even needed to show up.
Another change that made me wince was Theoden's initial refusal to help Gondor. The strong friendship between Rohan and Gondor always gave me a warm fuzzy feeling, especially when we hear about so many other alliances and kindreds that have become estranged. Plus it made one of my favorite characters seem kind of petty.
I think some purists were really worked up though by the confrontation between Gandalf and the Witch-king at the Gate. Whereas in the movie the Witch-king seems to have the easy upper hand (both practical and symbolic from the breaking of the staff), I think in the books they were either about equal, or Gandalf (the White) was the stronger.
All told though, I found the movies to be enjoyable, and I can understand some of the changes Jackson made when bringing the books to the big screen. Especially when compared to some of the other fare (*LOL* at SoP's link on the other thread!).
As for The Hobbit... bring it on! Smaug on the big screen. Heckyeah!