The heroine of the book is Fanny Price, a powerless and socially marginal young woman. To almost everyone she knows, she barely exists. As a child, she is sent to live with the family of her wealthy uncle. Her parents give her up without regret, and her uncle only takes her in because he is deceived into doing so. Fanny's wealthy relations, when they deign to notice her at all, generally do so only to make sure she knows of her inferiority and keeps in her place. Fanny is thus almost completely alone, the only kindness she receives coming from her cousin Edmund. Forced by circumstances to be an observer, Fanny is a faultlessly acute one, as well as the owner of a moral compass that always points true north.
Those who dislike "Mansfield Park" almost invariably cite Fanny as the novel's central fault. She is generally accused of being two things: (1) too passive, and (2) too moral.
The charge of passivity is perplexing. Surely it is evident that for her to challenge those in power over her is extremely dangerous - in fact, when she finally does challenge them, on a matter of the greatest importance to her and of next to no importance to them, she is swiftly reminded of the weakness of her situation by being deported back to the impoverished family of her parents, who receive her with indifference.
The charge of morality is easier to understand - many readers feel themselves being silently accused by Fanny, and they don't like it. The interesting thing is that those same readers often enjoy "Pride and Prejudice", even though it is evident that the same moral standards are in place in both books. So, why do readers feel the prick of criticism in one and not the other?
Part of the answer is that in "Mansfield Park" the stakes are higher, which squeezes out the levity of "Pride and Prejudice". Elizabeth Bennet, the heroine of "Pride and Prejudice", can afford to smile at the follies of others - they are not dangerous to her (at least she thinks not - she comes to think differently before the book is over). Fanny, however, can seldom afford to laugh. Vices that are funny in the powerless can be frightening in the powerful. Fanny's vulnerability to the faults of others is clear to her, and she suffers for it throughout "Mansfield Park".
Another part of the answer is that attractions that are combined in "Pride and Prejudice" are split in "Mansfield Park". In "Pride and Prejudice", Mr. Darcy is both rich and good; in "Mansfield Park", Henry Crawford is only rich. In "Pride and Prejudice", Elizabeth Bennet is both witty and good; in "Mansfield Park", Fanny Price is only good. Readers who liked "Pride and Prejudice" because it had a rich man attracted to a witty woman, will either find nothing in "Mansfield Park" to engage their enthusiasms, or, as is not uncommon, they will actually find themselves drawn to the book's sometimes-antagonists, the Crawfords.
Having dealt with why some people dislike "Mansfield Park", it remains to deal with why other people like it. Its central attraction is the skillful blending of the story of Fanny Price herself, which is the Jane Austen's adaptation of the "Cinderella" archetype, and the story of the other characters, which are of the great Christian themes of fall and redemption.
"Cinderella", is of course the story of hope for the powerless. It has been subject to a certain amount of well-intended misreading in recent decades, but the motive for that misreading really concerns an accident of the eponymous story - the sex of the main character - rather than its real theme, which is universal. "Harry Potter", for example, shows how easily and successfully the Cinderella archetype can be applied to a male protagonist.
Fall and redemption is the other story of "Mansfield Park". At the start, the characters other than Fanny are fallen or falling. Some are so corrupt that we are have no hope for them; their presence is purely malign, endangering those not so badly off as themselves. Others have fallen far, but are not quite so far gone that we do not have hope for them as well as fear of them. Finally, there are those who are only beginning to fall, whose danger is all the more alarming for it.
In "Mansfield Park", these stories are not just side by side, they are interwoven. Jane Austen's Cinderella saves not only herself, but also saves - and almost saves - others as well. All but the worst characters in the book are drawn to the goodness in Fanny, even while they yield to the temptations that threaten them. The book has real tension in that we don't know who will make it and who will not. Those who feel sympathy for the Crawfords are not entirely misreading the story - we are not wrong when we sympathize with a drowning man clutching at a rope thrown to him. Where we can go wrong is not when we wish not for the drowning man to be pulled to shore, but when we wish for the person at the other end of the rope to be pulled in after him.