Cold Days is one of the better stories in the Dresden series. Butcher manages to keep a really long series running pretty tightly, which absolutely cannot be said of most long-running series.
Things are getting more serious, and the stakes are getting higher, but not because of some sort of exponential increase in the powers or abilities of the people, just because now, you care about many of the characters. Peripheral characters are so much better known that when things happen to them, it really does matter.
It's actually kind of nice that Harry isn't getting smarter or better at an insane speed. Yes, he has more power because he's the Winter Knight, but when he removes that mantle, he loses quite a bit, and if he doesn't play by the rules, he also gets damaged. There are limitations, even to awesome.
A whole lot happened in the last few bits of the book, and I'm looking forward to watching the repercussions hit in the next book.
The only real complaints I have are:
1. The random political lecture we had to have (thanks Butcher, but... uh... more magic, plz?)
2. The expostion-ing. Seriously. This is book 14 in a series. We know how magic works, and I sincerely doubt that you need to worry about too many newbs picking up the first book 14 into the series.