This is not the most popular of Austen's novels, and Fanny Price is probably the least popular heroine. But then, popularity is not something she would value. To love Fanny one must begin, as Mr. Crawford found, by seeking to get her to like you. Once you have learned what a challenge it is to obtain her good opinion, you begin to realize how very worth it is getting.
Fanny is in some ways the antithesis of Austen's most popular heroines. The novel is a triumph of character over personality. Indeed, in personality and intelligence Mary Crawford is very nearly exactly like Elizabeth Bennett; but without sound principles, her charm must pall. Both Elizabeth and Emma are outwardly-engaged people who must make a journey inward--Elizabeth must learn to slow down and judge others rightly, and Emma to know herself.
But Fanny is already adept at understanding herself and others; she does not need witnessed explicit roguery to see Mr. Crawford's flaws, nor the appearance of a rival to recognize her own attachment to Edmund. It is, indeed, her own acute awareness of her own feelings and those of those around her that makes this novel the most emotionally intense and least light-hearted. Her whole life is one of observation and thought, intensely internal.
Fanny needs to journey outward: to stake the place for her own opinions and conscience in the world. To have the courage to hold on to her own "no," even against those whom she respects the most. In order for her to become Edmund's lover, she must cease to be his mirror, for all real love is a love of the otherness of someone. In order for her to be valued for her principles by Sir Bertram, she must hold true to them when he cannot see how they apply. To complete her journey she must become an entity in her own mind: realize her ability to mentor Susan, and recognize that her role at Mansfield Park is not simply a poor relation, but the capstone of the whole edifice.
Mr. Crawford starts out the least far along the path of vice of any of Austen's villains. His worst established fault is flirting with an engaged woman. Indeed, Austen raises the possibility that had he managed to persevere a bit longer and won Fanny, he might have been reformed. (Nonetheless, Austen seems to have little faith in the Love of a Good Woman as a motif and refuses to submit any of her heroines to marital reform work.) His real flaw is vanity--he cannot bear to *not* be admired, and that both attracts him to Fanny and undoes him with Maria.
An interesting contrast can be seen between the favor Crawford does for Fanny (securing a promotion for her brother) and the favor Darcy does for Elizabeth (forcing Wickham to marry Lydia). Crawford immediately proclaims what he has done, and then presses his romantic attentions on her--in the very same conversation. He obviously expects a quid pro quo. Fanny feels the obligation, but does not let it shake her principles. And note that although it was obviously a great benefit to William, the cost to Crawford was nothing more than a bit of gadding about and socializing--hardly a sacrifice for him.
On the other hand, Darcy, who must undergo considerable humiliation and expense to deal with Wickham, tries to keep the favor a secret. His love is, by this time, entirely unselfish--his only thought is that Elizabeth be benefited, not that he be credited or even that she be won. (And indeed he succeeds, not because she feels obligated to him, but simply because it confirms to her both his character and his continued regard in spite of the obstacles.)
A few more random thoughts--although Austen has many characters in running for the Person Most Desirable to Slap Silly, surely Aunt Norris is in the very top running. Also, Mansfield Park gives us a more intimate picture of young men than seen anywhere else--it is the only place where the main character has close familial relationships with them. And seeing the way Edmund and Fanny interact, it is not hard to understand why cousin marriages were relatively common--with the allowed intimacy of the family circle, it must have been the only opportunity to really get to know another person, rather than having to select one off the marriage market on the strength of a few balls and card parties and a frank tallying of assets.