Here are the major changes:
-A major alteration present is the more "stylized" and "outlandish" nature of the fight scenes instead of the more "gritty" and "realistic" approach taken by the novel. The costumed vigilantes more or less become "super heroes" as characters are seen punching through walls, inverting knees, jumping outlandish distances, and twisting heads backwards; to name just a few instances.
-Rorschach's first visit to Happy Harry's has been removed from the theatrical cut. Not entirely sure if the scene will make it into the directors cut or not.
- Rorschach's visit (warning) to Veidt has been removed and instead replaced with Dan Dreiberg visiting Viedt and warning him about the idea of a "mask killer" on the loose.
- Rorschach's commentary on Veidt with the line "Possibly Homosexual? Must Remember To Investigate Further" was removed from the theatrical cut.
- The two Bernie's seen on numerous occasions at the newsstand are given minimal screen time and no dialogue. Though scenes with them were shot and expect to see them restored in the directors cut.
-Laurie does not smoke at all.
- "The Crimebusters" are now referred to as "Watchmen."
- Capitan Metropolis is no longer a part of The Crimebusters meeting and is instead replaced by Veidt.
-Dreiberg's (Nite Owl II) impotency is not explicitly stated.
-Laurie no longer throws the bottle of perfume on Mars, but instead repeatedly hits the tower structure Jon built until it crumbles.
-Dr. Manhattan explicitly states that The Comedian is Laurie's father, rather than simply imply it like graphic novel chose to.
-The vivarium's been removed altogether.
-Hollis Mason's (the original Nite Owl) death was removed completely from the theatrical cut. No mention of it remains, although the scenes were shot and should be present in the directors cut.
-The fight scene between Rorschach, Nite Owl II, and Veidt has been altered to be more outlandish and stylistic than what the graphic novel originally portrayed it as.
- The scene where Big Figure and a fellow prisoner cut through the heavy set prisoner to get into Rorschach's cell was altered a bit to be more gruesome.
- Rorschach's backstory remains intact, for the most part. he still murders Grice's two German Shepherds and tosses them at him just like the graphic novel...
Here's a rather lengthy explanation of the altered ending and how it plays out...
- Veidt and Manhattan essentially team up to create a new form of energy. Veidt then uses Manhattan's signature energy to create numerous explosions in a select amount of major cities, although only New York is shown. Shot of blue explosion is displayed and the two Bernie's embracing to form the graffiti of the two figures on the wall remains. The world then assumes that the attack was emotionally driven by Manhattan due to the numerous media attacks against him (which were originally caused by Veidt, to begin with). Manhattan then decides to depart Earth in a similar manner to the graphic novel.
- The infamous scene in the graphic novel with numerous corpses piled throughout New York has been replaced with a landscape of rubble. Think of it as the aftermath of a large explosion, with no bodies present to provide the impact of the original scene in the novel.
-Rorschach's death is essentially the same, although Dan Dreiberg witnesses the moment to increase the dramatic nature of the scene.
Here's a rather rough run down of the final moments between Dreiberg and Veidt...
- Dan Dreiberg becomes enraged after he witnesses the death of Rorschach. He rips his cowl off and rushes inside to confront Veidt and delivers numerous punches. Dan wants Veidt to be the bad guy and attempts to solve the problem through fighting as a "hero." Veidt does not fight back and Dan realizes it's pointless and him and Laurie leave Veidt to ponder his actions.
- The sex scene between Dan and Laurie in Karnak has been completely removed.
And now for the final shot...
- The ending scene remains intact, with the exception of the addition of the line "Tonight, a Comedian died in New York" from Rorschach as the camera zooms in on Seymour's stained smiley face shirt. It's not explicitly shown that Seymour grabs Rorschach's journal and the open ended nature of the graphic novel's final panel still remains, albeit with the subtle nature of the reference to the Comedians stained smiley face badge ultimately being lost due to the addition of Rorschach's line.
Overall, It wasn't bad. Too much superhuman strength and slow-mo, though, and the omission of the dead bodies in New York really ruins the dramatic effect of the scene.