Pacing, mostly.
Also, in the first two films there were no quick solutions to anything. The green ghost wave washing everything away is just kind of underwhelming.
Also the action. Legolas sliding down a flight of stairs was impressive in Two Towers. In the next film he becomes Fed Flinstone, slighting up and down Elephant trunks killing 60 people at a time. Return of the King was the beginning of Legolas a cartoonishly unstoppable death machine, which reached ridiculous heights in Desolation of Smaug.
The fight with Urukhai at the end of Fellowship has so much weight to every blow, and Two Towers just had a sense of dread about it. The action in Return of the King was almost superfluous.