Ensemble Darkhorse: The Skandians. For a people that started off as just an Always Chaotic Evil Viking Expy, they end up getting a ton of focus. And the sequel series actually involves them more than the Araulen.
Fridge Brilliance: Halt taking honey in his coffee. The other three recurring rangers (Will, Gilan and Crowley) make it clear that it's considered incredibly weird, and nobody else does it. But Halt having a private sweet tooth makes sense. He grew up as the crown prince of Clonmel. So, unlike the other protagonists, he would have grown up having sweets on a near daily basis. He's more used to sugar than the other rangers. What's unpleasantly sweet to them is probably delicious for Halt. That's just how his palate developed.
Furthermore, lots of nobles practice archery when they're young. No wonder he's already an expert shot by the prequels-he's probably been practicing since well before he met Pritchard, who just "tightened up his technique."
Of course, as a yardslave you are very unlikely to last more than a month, let alone a year, but the Warmweed makes it worse by taking away every chance of recovery.
Growing the Beard: The series becomes increasingly well-written, original, and funny, with a major leap in stylistic quality, between books six and seven.
Narm: The Kalkara. They're actually described in-universe as bear assassins.
Nightmare Fuel: Warmweed. This is a plant that essentially mindwipes you after a few doses, is apparently quite common and easy to get, and was distributed to most of the Skandian yard slaves for a very long time.
The numerous Family Unfriendly Deaths in the series, for some people, leading into What Do You Mean, It's for Kids?. For example, the Kalkara in the first book are so dangerous because massive layers of hardened oil make them virtually invunlerable. When the characters get access to fire arrows, they ignite in blindingpillars of flame, ''and are completely disintegrated.
The barrow wight in Book 9 could also count.
Shown Their Work: Most of the author's explanations on archery, culture and battle tactics is generally correct, if occasionally somewhat simplistic. But it crosses into Nightmare Fuel when he writes a chillingly accurate portrayal of exactly what it's like to virtually die of dehydration, and just how easily it can happen to anyone who doesn't know the desert.
Tear Jerker: The Royal Ranger book begins with a time skip in which a lot of things have changed. Starting with Alyss being dead.