Angst? What Angst?: Lampshaded — Shasta learns in a single evening that the old fisherman who raised him is not his real father and that the same man plans to sell him as a slave. He's not dismayed about either of these, since he knows he doesn't love the old man and, after all, his life is hard labor anyway, so being a slave would be no different and possibly better. What bothers him is that the visitor's horse speaks up and tells him that being his master's slave would be a Fate Worse Than Death.
Broken Base: The use of Calormen as a Fantasy Counterpart Culture to the Middle East. It's been interpreted as racist by a lot of people, as the lone unambiguously good Calormene character (Aravis) still starts off as rather haughty and eventually becomes a northern princess. Others are more sympathetic, pointing out that only a few minor changes would have to be done in an adaptation to make the setting more appropriate for modern audiences.
Cult Classic: The Horse and His Boy is definitely the odd book out in the Narnia series, as it's not part of the primary plot line (instead serving as a side-story taking place during The Lion, The Witch, and The Wardrobe's late Time Skip), does not star any Earth children traveling to Narnia, and can be removed entirely from the series without leaving any holes. And yet, it has a very loyal following who like it because it's the Oddball in the Series. In fact, this is the book many Narnia fans are most excited to see get a film adaptation.
Fair for Its Day: This book is slightly more feminist in terms of the women fighting than The Lion, the Witch, and the Wardrobe's infamous "battles are ugly when women fight". Aravis is at least familiar with weapons and it's illustrated that she could fight in battle if needed - not to mention keeping her head at a difficult situation in Tashbaan. Likewise Lucy is said to lead the archers in battle.
Fanfic Fuel: As the only book in the series to take place entirely in another land - and we're given plenty of details of Calormene culture - it results in plenty of plots that explore more of what happens in Calormen.
Paranoia Fuel: What nearly happens to Susan is quite alarming. Imagine going on a visit that you assume is going to be diplomatic...and you realise that the prince who wants to marry you won't take no for an answer, and that you are now stranded in a country that may turn hostile with your friends and family, surrounded by his troops. Although she escapes safely, a war nearly gets started over Rabadash's obsession.
Susan opts not to fight in the war, other characters claiming "she's like an ordinary grown-up lady" who doesn't like fighting.
Calormen does call on some Middle Eastern stereotypes - as it's a Fantasy Counterpart Culture of "Arabian Nights" Days. While there are good characters like Aravis and Lasaraleen to balance it out, it's clear the book is from a time when such stereotypes were common.
Despite Aravis being knowledgeable in weapons and fighting, the plot finds a way to keep her out of the battle towards the end.