Fridge Horror: There are probably people with a Grace for sex. And Gracelings are usually enslaved by kings...
Of course that's the kind of Grace that would take (hopefully!) about sixteen years to discover, so an impatient king might declare them useless by then. As long as they're not stupid enough to go bragging about it, they should be fine.
Ho Yay: Raffin and Bann. According to Word of God, their canocity is her most frequently asked question.
Katsa: I told him I'm not going to marry you and hang on to you like a barnacle, just to keep you to myself and stop you loving anyone else.
Po: It's all right, you know. Other people don't have to understand.
Katsa: I worry about it.
Po: Don't worry about it. We'll muddle through. And there are those who do understand. Raffin does. And Bann.
Katsa: Yes, I suppose they do.
Confirmed as of Bitterblue.
Real Women Don't Wear Dresses: Katsa hates wearing dresses. Or anything feminine, really. Which leads to Broken Base: Refreshing subversion of feminine stereotypes, or novelized hatred of all things effeminate?
Justified Trope: Katsa's nature is that of a fighter, and she is most used to wearing clothes that are comfortable and suitable for fighting. Also, the fact that Randa forced her to pretend to be a lady while he toted her a weapon. On the inside she felt like a monster, not a lady, so it is natural that she would reject ladylike behaviours. The fact that Randa essentially used forced ladylike behaviours and clothing as a method of controlling her would also naturally lead to resentment of all things feminine.
Some Anvils Need to Be Dropped: Part in parcel with the Broken Base. While there are those who don't like the vehement rejection of marriage and motherhood that Katsa displays in the book, many have expressed appreciation for a female lead who not only averts the standardized Happily Married and Babies Ever After ending, but firmly rejects it. A woman who flat-out does not want children and has zero desire to be a mother is often considered odd even in today's world, sometimes to the point of encountering hostility regarding her decision and views, so there are those who found it gratifying to finally have a female protagonist who was not only firmly against personally having children, but did not fall prey to the Mandatory Motherhood trope.
What Do You Mean, It's for Kids?: Despite the 14+ rating, there are a couple of (nondescript) sex scenes in this novel. This has become a subject of controvery among Amazon reviewers.
Subjective tropes in Fire:
Mary Sue: Fire herself, to some people. Of course, she's probably a deconstruction, as the novel points out the Real Life consequences being a Mary Sue would have. She is also called out on her flaws and/or on her mistakes by other characters, including at least one Quit Your Whining scene.
Real Women Don't Wear Dresses: In direct contrast to Katsa, Fire subverts this. She's a musician, desperately wants children, does indeed wear dresses, and is mentioned to like flowers and flower arranging. And she's not much of an Action Girl either, although she tries. However, she's hardly a wimp.
What an Idiot: Brocker? You shouldn't have shacked up with your mad king's wife, okay?