There wasn't anything seriously wrong with this book, at least not that I can put my finger on. Theoretically it had plenty in it (both plotwise and characterwise) to make it interesting.
Unfortunately I just couldn't really get into it. The characters left me cold, and I simply wasn't interested in what happened to them!
I can well understand why it's got a range of differing reviews here. What I would suggest is - download the sample and read that (if you're reading on a kindle), or read the first few pages in the bookshop. If it grabs you, great, you may well enjoy it. If your response is more along the lines of "meh, but wonder if I should give it a go (possibly based on the back of her "visit from the good squad", which has such good reviews, but which I have not read), I'd suggest don't - there is no dramatic improvement.
I'm glad I bought it when it was the kindle deal of the day, or I'd probably have been very disappointed. Shame it's now put me off trying her other stuff.