A good narrative without descending into silliness at any point. The story developed slowly, with more emphasis on the peripheral characters than on the central figure. This made his scenes even more powerful. Very few Hollywood clichés were employed so it was never clear which way most characters would jump.
And finally there was a strong, involving and highly satisfying ending.
Whether or not they decide to try for a second series is chiefly irrelevant. Personally, I hope they don't. But if they do, I shall be in the queue to watch it.