What makes Leliana so amazing is that she's the real deal. She doesn't have to pretend to be something that she's not, she doesn't have to hide insecurities behind professed certainties. She can afford to be genuinely curious when she discusses questions of faith with Morrigan instead of feeling threatened and going defensive. She doesn't have to use platitudes and smart-aleck evasion as a shield.
Well, that and her phantastic voice actor who brings her to life as the genuine article.
Alistair reminds me of some people I know in RL, who try very hard to think what they think they ought to think, until they convinced themselves that, yes, their wild years are behind them and what they truly want and what makes them really, really happy is to be the perfect husband, father, believer or whatnot. Until the slightest amount of pressure is brought to bear. Then the whole house of cards collapses and they disintegrate, jumping straight to the bottom of a bottle. Just like Alistair.
The reason is that shallow, superficial attempts at emulating something rarely have significant effect. If there is a discrepancy between what we are (feel, want) and what we believe we ought to be then we have to look deep and grow; something has to change until the discrepancy is gone. If won't happen overnight but resolve can bridge that gap. Pretence resolves nothing and prevents growth.
That reminds me of my little brother in his early teens, when he thought that sweeping was about waving a broom around in a manner resembling what he had seen, and that washing up consisted in getting the dishes wet and farting around with a cloth. He had completely missed the point about removing dirt, and consequently everything was just as dirty after as it had been before.
At the surface, Alistair is lawful good. But beneath the surface he is just a spoilt little boy, shallow and totally self-centred. Since his 'convictions' go only skin-deep they can be discarded in the blink of an eye if he is not given his toy. Be it tent-time with the Warden or the immediate demise of Loghain, right then and there at the Landsmeet.
That's the big difference between Leliana and Alistair. It is easy to respect and like Leliana without reserve, even if you do not agree with her.