He really grew as a character throughout the games. He's one of those NPCs who end up being even better than some main characters.
As a woman, it's as if I were watching the maturing process of a young man. Cullen evolved from an adorably naive young man (DA:Origins) to a righteous and somewhat rigid compromised soldier (DA2) to a wise and accomplished warrior who knows the truth is not so black and white (DA:I).
He goes through the 3 main stages of maturity. He was literally shaped by the circumstances to become a leader. As fans, we can see how the events in the 3 games made him the confident and capable man he is today. His confidence stems not from arrogance, but hard decisions which forced him to stand his ground. His ability to command came from seeing those superior to him fail, forcing him to take charge and avoid situations getting out of control.
And what's best: Cullen went through hell - imprisoned and tortured by blood mages, killer of innocent mages, confronted a mad leader - and survived. As a matter of fact, he did better than just survive! Hell, he was supposed to have been destroyed in the process. But no amount of suffering managed to leave him a broken man. Rather, the hardships strengthened him and made him find new vigor inside himself. He emerged a new man, a new Cullen, wisened, experienced and fully aware of his capabilites, ready to take charge in the face of a crisis.
The hesitant and idealistic templar blossomed into a fine man, indeed! He faced his own fears, his own shadow - in the shape of Uldred and Meredith - and emerged stronger and victorious.
How more exciting can this be?
And to top it all, he was at the heart of the mage-templar conflict, which originated all the chaos that is consuming Thedas right now. He has a large amount of expertise that will be just perfect to guide the Inquisitor through the hardest decisions.
Boy, I sure hope Bioware worked a lot in his romance quest. The producers are more than aware of the character's large fanbase. They did a fine job with him, developing his personality beautifully. Don't screw everything so near the finish line.