Not that I nurse a grudge, but here’s the list of things Peter Jackson’s Lord of the Rings movies got wrong just by leaving them out:

Tom Bombadil 
Barrow Downs
Glorfindel
The Scouring of the Shire

Here’s the list of characters changed:
Frodo is too young.
Aragorn is too nice
Arwen is too present
Galadriel is too…um…plain.
Faramir is too rotten
Sauron as a burning effing EYE ON TOP OF THE TOWER.

…And the list of things exaggerated:
Legolas being able to run on top of snow turns into skateboard moves on shields, on stairs, and on the trunks of oliphaunts.

The role of the Army of the Dead – they help capture the ships, discussed in a flashback in the novel. If we had 20 minutes of time to film this and to extend it with piles of skulls for the extended version, why didn’t we have time for anyone on my first list?

Faramir capturing Frodo and dragging him back to Osgiliath.

Goblins and orcs – really exactly the same species. Different teminology used in The Hobbit and TLOTR.