There are a lot of things that can be said about this film. I think this calls for a list.
1. I'm of the opinion that brutal material should be brutal to watch. If you are going to make a film with gruesome sexual abuse and violence, I want to feel the horror. If you go around only implying it, it doesn't have that effect. This is why the Hunger Games films will be terrible. It is a book series that describes really horrifying things, meant to be horrifying. Once they are made palatable, it won't succeed at making the point. The lack of pulling punches is one reason Irreversible works so well. And in a different vein, Hunger works so well because it is so visceral (especially the walls of the cell). I'm not saying Salo needed to show everything exactly the way it is here, but it needed to be horrifying.
2. I find this film to have an interesting context in a world where videos of people's reaction to watching two girls one cup (don't google it) was an internet trend. There were parts of Salo I found less tolerable than that video (which I maintain was equally fake), but to the degree that people have seen that and tolerated it at all, they can probably handle Salo.
3. At many points of this film, it could almost be an elaborate BDSM group party. Power games, bisexuality, polyamorous, and a few unspeakable kinks. If you listen to Dan Savage's podcast enough, there are people out there with enough...different...tastes that it could only seem plausible. But that setting would have a sense of mutual respect and safe words, which leads us to:
4. Fascism. At the start you get the feeling that the nine young men and nine young women were selected because they were tied to people who had in one way or another threatened those in power, this being set amid fascist WWII-era Italy. Threats to family have always been an effective means of control. You also get the concept of fascism and its total control as a form of slavery that reminded me of early scenes in Spartacus, though with more potency. This nearly complete lack of free will. There is one other thing that struck me particularly as relevant to fascism/totalitarian regimes but I don't wish to spoil it.
5. Dante's Inferno. There are hints of Dante in this story and its circles, though it is not a rigorous interpretation of that story. It does provide a bit of structure to the process. I'd almost have wanted more circles because it spends a bit too much time on individual elements.
6. Naked bodies get boring. In terms of number of people, male and female, and amount of screen time they are naked, this film would rank rather high. While the grim nature of the film is enough to dampen the potential sexiness involved in young naked people, simply the excess of the exposure tends to work alongside the increasing darkness of the film to remove any such context.
Apparently the Marquis de Sade book upon which this is based, 120 Days of Sodom, is actually more gruesome than this so I suppose one has that to be thankful for in watching this film. I think there are a lot of really interesting ideas going on here, though the main problem came in it feeling like it gets caught in ruts from time to time. Still, a really fascinating work that has me interesting in further films from Pasolini.