Doran (Alexander Siddig), who ruled Dorne, was killed by the Sand Snakes within the first half-hour of the premiere. If you've only watched the HBO show, that was probably no big deal. But if you read the books, you're probably fuming.
As we understand it, Doran had a much more significant role in Martin's novels. He had a long-standing plot to marry his son (whom the Sand Snakes also killed) to Daenerys. And to make the kill even worse for book fans, the Sand Snakes wanted to crown Myrcella Baratheon their queen. So it doesn't make sense that they'd kill her and kill the Martells.
Clearly, the showrunners are veering from the books on that one, and the purists aren't happy.