1. The most obvious difference is that of running time: while the films are very similar structurally, the 2005 film is an hour and a half longer than the 1933 film.
2. There is much more backstory given to the two principals, Ann Darrow and Carl Denham. Ann did not appear in the 1933 film until the point at which Denham covered for her attempted shoplifting of food.
3. The character of Denham is quite different. He was originally a successful, respected filmmaker. The newer film portrays him as a man on the outs with the studio heads who steals the footage for his own film and flees after learning the bosses are planning to scrap the picture. The newer incarnation is also played as much more of an antihero; smarmy but likable. He is motivated more out of desperation than greed, whereas the 1933 film never brought the character's actions into question.
4. Jack Driscoll is now a playwright who is working on Denham's screenplay; previously, he was the First Mate of the Venture.
5. The natives of Skull Island are much more hostile. In the original film, they allowed Denham's party to leave without incident; in this version, they kill several members of the group before the Venture's captain arrives with help.
6. All references to and quotes from Conrad's Heart of Darkness are new to this version of the story.
7. Kong is played less as a monster and more as an animal. Whilst in the 1933 version they portrayed Kong as a unique gorilla-like ape, this Kong is purely a giant silverback gorilla; its behaviours are very accurate to silverbacks.
8. The 2005 film contains a scene from the screenplay and novel of the 1933 version, which never made it into the film(whether this scene was shot or not is still up for debate). Originally, stop-motion spiders attacked and ate the crew members who fell off of the log bridge and into the gorge it spanned. The new version presents a variety of insects and leech-like creatures, but eschews spiders, possibly to avoid feeling like a retread of Jackson's previous film, The Return of the King, which had featured a battle with a huge spider.
9. In the 1933 film, Ann Darrow remains frightened of the giant ape throughout the film. In this version, however, she and the gorilla form an emotional bond, with Ann recognizing human traits such as intelligence and empathy in her captor-turned-guardian. This is not the first time a Kong film took this approach, however, as the 1976 Dino De Laurentiis King Kong had the lead female character, Dwan, show deep respect and sympathy for her simian captor.
10. Ann and Jack originally joined Denham on stage as he presented the captive Kong to a New York audience. In Jackson's film, both have become disillusioned by his glory-minded pursuits and decline to take part. Denham therefore hires an actress whom he tells his audience is Ann, and he credits her rescue not to Jack, but to the actor Bruce Baxter, who had actually abandoned the search for Ann to save himself.
11. The New York rampage in the 2005 version is more low key than in the 1933 Kong and not nearly as horrific, most likely to keep the PG-13 rating. The attack on the elevated train does not occur in the new version, nor does Kong throw a woman he mistakes for Ann to her death-at least not definitively; he does throw a number of women about casually, but their fates are not depicted. Likely a few bumps and bruises, at worse a broken bone or two, but they certainly weren't thrown from a height or with enough force to kill them.
12. Now, Jack is the only one to race to the top of the Empire State Building to save Ann at the film's climax. Previously Denham and a few other rescuers had also ascended the skyscraper.
13. Kong's death is no longer seen as the necessary destruction of a dangerous monster, but the tragic killing of a noble beast who has suffered at the hands of human exploitation.