I agree with TheAbominatrix, completely.
After losing her father in the fight against Sin - a sacrifice that brought Spira merely a few years of peace - Yuna decided to commit to becoming a Summoner, to making the same pilgrimage and the same sacrifice. That takes enormous strength. She trains and learns and studies, gaining the power to save the world by ending her own life at the age of 17, when most people have barely begun to live. Even on her journey, she remains upbeat, trying to bring happiness wherever she can. Even when she's surrounded by death and suffering, like in Kilika and after Operation Mi'ihen, she tirelessly gives her all to ease suffering. And the entire time, she knows that every step she takes is taking her toward her death, a death that'll keep others safe for a short time, and earn her a statue in a few temples.

In short, there are few characters in the game as brave and selfless as her.

Personally, I liked the mixture of sadness and determination, the bittersweet emotions she carried because of her chosen path. Only once is she seen to cry during the game, and those tears come because she can't stay with the man she's falling for: she's upset because her own goal will deprive someone else of his dreams and happiness. Instead of being miserable about her own doom, she tries to seize the moment and enjoy her pilgrimage as much as possible. Even when new hope appears, and it seems that her sacrifice is no longer needed, she remains on the front lines, battling to free Spira once and for all.

As for her outfit, I consider Yuna the best-dressed female in any FF game, with that unique blend of traditional Japanese and contemporary styles that she wears in FFX.

I may be almost alone in this, but I adored Hedy Burgess' voice acting, too. Yuna's mix of vulnerability and conviction shone through really well, the melancholy aspects of her persona as well as her brighter side. Even if she had to speak at an odd tempo because of lip-synch issues.