Racism. It takes B. B. Underwood's newspaper editorial concerning the death of Tom Robinson to make Scout fully understand the social prejudice that Tom faced during a trial in which he is charged with raping a white woman and being judged by an all-white jury.
... Mr. Underwood's meaning became clear: Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men's hearts Atticus had no chance. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed. (Chapter 25)
Inequality. Scout is upset when Aunt Alexandra won't allow Walter Cunningham Jr. come to the Finch house to play. She doesn't believe he is good enough to associate with Scout
"Because--he--is--trash, that's why..." (Chapter 23)
Loss of Innocence. Scout and Dill's meeting with Dolphus Raymond is a memorable one: Dolphus reveals his secret about what's actually in the bottle hidden by the paper sack, and
I had a feeling that I shouldn't be here listening to this sinful man with mixed children and didn't care who knew it, but he was fascinating. I had never encountered a being who deliberately perpetrated fraud against himself... (Chapter 20)