(Semi-spoiler alert. I'm on page 175.) I had been curious about what exactly happened when Mrs. Weasley called Harry down to the laundry, and had him pick up a certain sock under the pretense that she was sorting socks. I thought maybe that was some kind of trace so the worried mom could keep track of her kids. But I should have been more "Mad-Eye" about it. Could the Death-Eaters be tapping into Mrs. Weasley's trace? Or worse yet, is Mrs. Weasley under the Imperius curse? Is that the leak, Snape's mysterious source? That would be perfect -- her inquisitiveness about the kids' plans seemed like motherly concern, but that would be a perfect cover. Ouch. That's wicked!
It was interesting to see Viktor Krum again, and get some history on him. Good to know that even at Durmstrang, which was portrayed as a generally dark school, there is a strong culture against the forces of darkness. Will we see more of Viktor? I expect so. It will take everyone coming together to win this.
But why was Luna's father wearing that dark sign? The Lovegoods and their (only seemingly?) crazy theories certainly have a role to play too, I suspect.
This book is far more terrifying than any of the others. The "kids" are adults now, and there's no cocoon of Hogwarts around them this year, no protection of Dumbledore, nor even a safe place to sleep or rest. It's a quantum notch more frightening.