My thoughts....
~ I felt that Armand had a suspicion all along that he was of mixed heritage. I feel that's why he was especially cruel to his slaves. He seems to be a person who takes pride in power and control (he refers to his family name as the oldest and proudest in Louisiana) and feels he needs to exercise that control over his slaves. He was kinder with the birth of his son, but after finding the letter his mother wrote and realizing he could no longer deny his identity, he became cruel again.
~There is some symbolism in the book. The outside of the home reflects that the estate is not what it had been in the past. This reflects the darker mood of Armand, who treats his slaves cruelly and is not fair to his own wife.
~Chopin can be very subtle with how she describes characters. One telling passage that jumped out on me after I read it a couple times was the description of how Armand fell in love with Desiree. "That was the way all the Aubignys fell in love, as if by pistol shot." Its possible this foreshadows that Armand's father had fallen in love with a slave and had a child with her.
~Desiree gets treated very badly for a character who is innocent. She is abandoned as a baby, enters into a marriage with a husband she fears (at least when he is angry) and is essentially thrown out of the house when her husband realizes that he is of mixed race. Desiree, as far as I can tell, hasn't done anything wrong to warrant these "punishments" (one could argue if being abandoned is a punishment, as she did end up in a loving home). I think that the racial theme in the story is the most evident, the theme of gender inequality may be the bigger theme. Even though Desiree denies it (and most likely is correct, though her racial makeup is never identified other than she is whiter than Armond) she can do nothing about it other than meekly leave the plantation. She is shamed by the accusations and does not go back to her mother, but instead into the wilderness and is not seen again. Desiree felt that even though she did nothing wrong, she could not face society. Chopin points out both racial and gender inequalities in this story and also shows how the two can overlap.