What happens when you take the most disturbing book of all time and try to make a movie of it? Typically, you fail. Pier Paolo Pasolini, however, was able to capture the spirit of The 120 Days of Sodom and add to the depravity, sense of despair, and the abject cruelty of the "Four Friends." If you are not familiar with Sodom, I did a brief overview here.
In Sodom, much of the focus lies in the stories being told by Duclos. The libertines are cruel, to be sure, but they are border-line comical. Sade is mocking them, and since the most horrific actions occur in the chapters of the book we do not have, they are not as feared as might be warranted. Pasolini changes this. Some other things make the movie more troublesome, and it is not purely the visual representation that does so.
In Sodom, the major action takes place a the Chateau Silling, which is high in the alps. It is impossible to get to after the libertines arrive, and they brick all windows and doors to prevent ingress or egress. It is winter. The chateau feels small. It is claustrophobic, and the libertines relish the seclusion. In Salò the chateau is out in the open. It is sunny and seems like Spring. Sade's vision may be darker, but the impression of things being in the open is more disturbing. Pasolini was apparently inspired, you might say, by the fascist activities in the country of Salò. In that case, the libertines were the law and could do these things publicly. This is more disturbing to me than clandestine actions.
Another area where Pasolini adds to the depravity is with his expansion of the role of the collaboratori, or "The Fuckers" as they were described by Seaver & Wainhouse. In Sodom, they are often mentioned, but have little in the way of personality. They participate, but are not the focus. In Salò, they are represented as young stormtroopers. They are armed, and often take action without prompting, though they are acting in the libertines' interests. They are still the libertines' lovers, but have an enhanced fuck-anything-with-a-pulse character.
The last thing I'll mention here is the expansion of the slaves' character. In Sodom, the slaves are primarily nameless, faceless objects. They have names, of course, but the names lose meaning, any thing that happens to one of them could be happening to any of them. If we had the whole story, there likely would be expansion of their characters, but they are undeveloped by Sade. In Salò, some of the slaves begin enjoying the depravity. Sade touches on this, but Pasolini expands the thought. It is plain that children in harsh circumstances will often be drawn to what appears to be right. Also, at the story-telling sessions, you see many sitting around listless, crying, bored... Acting as children would in situations they are not pleased with. This aspect comes across more effectively in visual media.
IMDb Entry for Salò o le 120 giornate di Sodoma