In my opinion this is a turning point in the series in which the story becomes a bit darker. This makes sense as the characters are a year older in the book and closer to adulthood. The actors too are a year older and their voices have changed as have their appearances as they are another year through puberty. It's also the first movie in the series to be rated PG-13. The first three were rated PG.
Despite the change in movie rating I was fine with my kids watching it. Next up: Harry Potter and the Order of the Phoenix. Hopefully it won't be another year before I write about this book/movie, but it very well could be given we are reading a different book first before starting the next Harry Potter book. :-)