Wow, it's been a long time since a fantasy book affected me that much. Especially a fantasy book about dragons.
It's because Novik knows how to create good characters. Strangely, her main human character is deeply loyal and empathetic to his fellow humans and dragons, but he lacks... enjoyment in things. In other words, he's a little bit too austere to be completely likable. I believe Novik has done this on purpose, to create weakness in her main character, but I'm not wholly convinced it has worked the way she wants it to. It's true that she's created that as well in her main dragon, who's just a touch too bloodthirsty and doesn't quite get the concept of loyalty to king and country right off the bat.
Novik worldbuilds well - the Napoleonic Wars with "aviators," that is, teams of men and women who ride dragons - and provides just the right amount of tension between the good aviators and the bad aviators (on both sides of the Channel). When the person you end up hating the most gets a bit of what's coming to them, you want to fist pump the air. That's surprising because I've read a lot of stories like this and not been as moved by these moments as in this book. This could be the effect of the book's voice - posh, and definitely 1800s, and building the plot slowly in layers - which belies its power in terms of storytelling.
I keep forgetting I finished the book, and get excited that I'll be able to sit down and read more of it, followed by deep disappointment. Fortunately, there are 8 more till the conclusion!