Mansfield Park has become famous in recent decades as a critique of slavery. Part of the plot hinges on Sir Thomas's journey to Antigua to personally visit his plantation, a plantation run by slave labor. When he returns, apparently altered by the experience, Fanny wants, but fears, to question him about slavery. Similarly, through an offhand, throwaway comment, Maria Bertram airily dismisses the disgraceful hovels on her fiance Rushworth's grand Sotherton estate. These hovels are passed by quickly--but Austen includes them for the astute reader to see. They stand as a critique of a system in which landlords spent vast sums on picturesque landscaping (something that becomes a subject of conversation around dinner tables in Mansfield Park) but won't spend the least amount so that their tenants can have decent dwellings.
Fanny herself is treated as a virtual slave by Lady Bertram and her sister Mrs. Norris. Mrs. Norris, especially, runs her around mercilessly. In one scene, Fanny is forced to cut roses in the hot sun, while Lady Bertram and Mrs. Norris sit in the shade because the heat is uncomfortable even for people who are doing nothing. Fanny ends up with a "sick headache" and Edmund is furious, although unable to get his mother to conjure the most basic understanding that if she was suffering in the shade, asking somebody else to toil in the hot sun was perhaps a cruel and insensitive demand. Austen here shows the ways lack of imagination can lead to abuse. It doesn't take an astute reader much to leap to what the slaves must have suffered on the brutally hot plantations in the Caribbean.
Further, the book critiques a marriage "market" in which women are routinely sold to the highest bidder. Maria Bertram is essentially sold off by Mrs. Norris, who brokers the marriage, to the wealthy but not very bright Rushworth. Maria despises him, and her father has doubts about the wedding, but all of these are repressed in the pursuit of wealth and status. Maria will, in the end, rebel against her marriage with unfortunate results. Likewise, the family is anxious to marry Fanny off to the wealthy Henry Crawford, and tries to coerce her when she resists his proposals. Money and status in their eyes--and society's eyes--trumps love. Likewise, Mary Crawford's downfall comes from her inability to accept that the man she loves, Edmund, really wants a career as clergyman that is, in her eyes, lacking in social status. Society has "deformed" her so that she can't accept love without high social status. Austen will never advocate for a wild "love" that breaks social barriers, but she does argue throughout her novels for marriage based on mutual respect, love and compatibility. She condemns sacrificing a woman's chance at happiness solely to money or status. She condemns selfishness in this novel about all the ways society has institutionalized and normalized selfish behavior.