I like that Jim knows the rules of his universes (Codex Alera included; I'm assuming the Cinder Spires as well, but we don't know enough yet to point them out), and sticks to them. When you ask a question about how something works, he knows the answer without having to think about it. One of the things that I didn't like about Harry Potter and other fantasy works is that magic often runs on whimsy, to the point that "a wizard did it" has become a meme. And it's not Dragonlance Vancian magic, either, with spells that are written down and studied in advance; it's based on principle of energy, and the way a wizard taps into, interprets, and employs that energy is unique to each practitioner, but the rules that govern such actions remain consistent.
I also love the characters—pretty much all of them, with few exceptions (I'm not a fan of Lash, for instance, which puts me in the distinct minority, and some of the minor characters (Andi, for example) are a little one-note. Also, Georgia needs to do more without getting kidnapped). I have a soft spot for Michael especially, and find every book in which he appears to be better for his presence. Skin Game made me actively shout in delight when Uriel showed up. I love watching Harry interact with most everyone, especially back-sass.
For me, the plots of each individual book are mostly excuses for Harry to do clever things while talking to people. The plot of the series is getting better as more is revealed, and the scope of the overarching conflict is growing grander as the stakes are raised. And the details we're starting to get make rereads of the series more interesting; Cold Days especially changed my perspective on pretty much everything that came before it.
For a series with the basic premise of "Die Hard meets D&D," I certainly didn't expect the narrative, character, and universe depth that I got. Plus the series got my dad to start reading again, so that makes me happy.