Another big thematic difference I think:

The film series, particularly 2 & 3, are essentially war movies. The books much less so, being more about what was going while the war was going on.

Which isn't a problem - I think that's a reasonable alternative take on the story, and probably works better as a movie. Except that - particularly with the portrayal of Theoden and the Ents - the message comes across as "Stupid pacifists - war solves everything" rather than as in the books "war is terrible, but sometimes not fighting allows even worse things to happen".