Adaptation Displacement: Most people in the USA have either only heard of Cruel Intentions or the 1988 Dangerous Liaisons film. Despite it being a French classic, the book is relatively unknown to the average non-French person. Indeed, the 1988 film version of Dangerous Liaisons was (for all intents and purpose) the first major exposure Americans had of the novel, which while popular in Europe, had never been given a major US release.
Do Not Do This Cool Thing: As much as you are supposed to see these pompous, virgin-corrupting, manipulative sex addicts as the bad guys... you can't help but admire them and perhaps even subconsciously absorb a little of their experience in the love game. Moral Guardians have been accusing the author's intent of being this since the book was published.
Magnificent Bastard: Merteuil, at least before her jealousy causes her to go too far, is on par with Iago in her ability to play everyone around her like a fiddle while making her petty spitefulness come across as awesome rather than pathetic. Valmont falls short of the mark because, even though he is an accomplished and manipulative charmer in his own right, he is essentially just an agent of Merteuil.