Broken Base: Of a sort. Book 12 got a mixed reception, to say the least. Some shrug it off and say it's not so bad, others are very displeased. Complaints include Alyss' death, the abrupt unlikability of most of the main cast, the rather poor handling of mental health issues, and the Darker and Edgier tone of the book as a whole.
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."
Also, Horace's knighting is a much bigger deal than may be immediately apparent to the modern reader. In our world, squires were usually knighted by the knight they served under, meaning that an Araulen apprentice would presumably be knighted by the Battlemaster or one of the Battleschool instructors. Getting knighted by the king is, in and of itself, a huge honour, let alone the fact that he's being knighted early and assigned to the Royal Guard.
In Book 12, Halt brutally condemns Will for wanting to focus on finding Alyss' killer over his orders from the top, despite having done the very same thing in Book 3, and when the country was in much greater need of all hands on deck. On the other hand, Halt ensured that someone would take over his responsibilities instead of just ignoring the country's need entirely, and his mission was also to benefit Araulen by bringing back a highly-talented Ranger apprentice.
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 invulnerable. When the characters get access to fire arrows, they ignite in blindingpillars of flame, ''and are completely disintegrated.