Mercy versus justice
The whole trial scene turns on this conflict. Portia argues that mercy is a higher value than strict legal justice. But then she uses the law to crush Shylock completely. The play doesn't resolve the tension cleanly, which is what makes it worth arguing about.
Prejudice and belonging
Shylock is excluded from Venetian society because he is Jewish. The Christians mock him, his daughter abandons him, and the court strips him of his identity. The play shows how prejudice operates through social exclusion, not just open hostility.
Appearance versus reality
The casket test is the clearest version of this theme, but it runs everywhere. Portia disguises herself. Jessica hides her plans. Antonio hides his feelings for Bassanio. Characters who trust surfaces get burned; characters who look deeper tend to succeed.
Wealth and obligation
Money moves every relationship in the play. Bassanio needs it to court Portia. Antonio risks his life to get it for his friend. Shylock's bond turns money into a matter of life and death. The play keeps asking what people owe each other when money is involved.
Law as power
The law in Venice is supposed to be neutral, but it consistently protects the powerful. Shylock uses it as his one tool for equality. Portia turns it against him. The play shows that law reflects whoever controls it, not some abstract fairness.