I think it's important to go into the Casual Vacancy with proper expectations: this is not Harry Potter or anything resembling a fantasy; it's a gritty, British social drama.
Rowling showed her strengths in this novel by creating a setting that felt like a place I'd really been, and building characters that felt like people I knew in real life. Not since reading War and Peace have I stopped during the day to wonder about a friend or acquaintance, and realized I'm not thinking of an actual person, but of a character in the book I'm reading.
I think the ending of the book needed a little more time and space to resolve. It felt less realistic and engaging when so many story lines came to a head all at once. That said, I thoroughly enjoyed reading this book and can't wait to see what Rowling writes next.