What are some of the biggest changes from the book to the movie?

To keep this within the 'biggest changes,' I'll keep it within the five greatest differences, though the answers being somewhat broad and general are a natural consequence of the scope of the question:

• There is less emphasis placed on the hobbits in the film, and more emphasis on other characters. Likely as a consequence of this, the Scouring of the Shire was cut.

• The 'Blockbuster-ification' of the story. The rushed pacing, the greater emphasis on action and battles, the ridiculous eye-candy monsters, etc.

• The changes made to Aragorn's character. It's a complete reversal in nature. Just about every character saw significant change, but few come close or exceed the changes to Aragorn. His greater prominence in the story than someone like Faramir means he get to be placed on the list.

• What can only be called Jackson ignoring the setting of Tolkien's story. The geography is greatly reduced in scope, time and seasons aren't regarded at all in the films and the cultural details of Middle-earth are ignored. Why is there plate mail everywhere?!

• Thematically, the films do a poor job of capturing the book. They get most of the more 'superficial' themes as a natural consequence of keeping the overarching plot generally the same, but they fail to capture the subtle and sublime themes of the work. Altogether, I got the impression that Jackson was not overly concerned with themes.