Harry Potter movies just keep getting darker and darker, with Harry seemingly less and less master of his own fate. Humorous and whimsical scenes are on the wane, compared to previous entries in the series. The usually-omnipotent Dumbledore seems to be getting in over his head. Even the weather at Hogwarts (as Mrs. Salad pointed out) adds to the mood; when it's not raining, it's snowing.
And the body count is starting to get pretty high for the good guys too.
Nevertheless, the movie never lags, with the usual impressive special effects and a tense and twisty plot.