In addition to love, I think there is a theme of the importance of marriage in the story. I realize that the two are closely related, but I feel it is important to note that the implied culmination of Westley's and Buttercup's love for each other is not sex. It's marriage. The theme of marriage is shown elsewhere in the story too. Humperdinck might be the bad guy, but he does understand that marriage to Buttercup is key to their relationship.
"I think, sweetest child, that we should strike a bargain, you and I: if Westley wants to marry you still, bless you both. If, for reasons unpleasant to mention, his pride will not let him, then you will marry me, as planned, and be the Queen of Florin."
Even Miracle Max and his wife can be used as evidence for the book showing the theme of marriage. They might argue and bicker with each other, but they remain committed to each other.
Friendship and loyalty were discussed by the previous post, and I'd like to add to the loyalty theme. The previous post discussed loyalty between friends and lovers, but I would like to add that the story also shows loyalty to family. Inigo's entire reason for his hunt and turning himself into a great sword fighter is to avenge his father's death. Inigo is even offered wealth and power beyond his imagination by Count Rugen, and Inigo tells Rugen (and readers) that what he wants more than anything else is to have his father back.
"I WANT DOMINGO MONTOYA, YOU SON OF A BITCH."
The theme of family and loyalty to family is also shown through Goldman discussing his life, the book, and his family. Readers are told that the book was important to him as a child because his father read it to him, and now he is passing the torch.
And my father read it to me. And now I give it to you. What you do with it will be of more than passing interest to us all.
Since I brought up Inigo and his motivation to become a great swordsman, another theme that has to be mentioned is the theme of revenge. Inigo absolutely wants his father back, which he knows is not possible. The next best thing is revenge. He needs to kill the six-fingered man.
"Hello, my name is Inigo Montoya; you killed my father; prepare to die."
Despite being entirely focused on getting revenge against Count Ruben, Inigo is willing to be patient. He knows that his opponent is an incredibly skilled swordsman, so Inigo knows that he must devote his life and all of his energy to training himself for the final confrontation. Inigo wants to try and get revenge, but he also wants to be successful in his attempt.
"Am I ready? If you say I am, I will seek him through the world. If you say no, I will spend another ten years and another ten after that, if that is needed."
Finally, there is a theme of society and class divisions. This theme is most noticeable through Humperdinck's character. He won't even walk among commoners, and he initially turns his nose up at the thought of marrying Buttercup.
"A milkmaid . . . I don't know that I could wed one of them even under the best of conditions. People might snicker that she was the best I could do."
Even Buttercup isn't immune from seeing herself as "better" than another person because of birth or societal role. That's why she initially doesn't even consider Westley as a potential future husband. She's not wealthy, but he's a lowly farmhand, and she doesn't let him forget that fact. Buttercup doesn't even refer to Westley by name when readers are introduced to them.
The farm boy did what she told him to. Actually, he was more a young man now, but he had been a farm boy when, orphaned, he had come to work for her father, and Buttercup referred to him that way still. "Farm Boy, fetch me this"; "Get me that, Farm Boy—quickly, lazy thing, trot now or I'll tell Father."