Confining myself to the OT, some major themes seem to be:
1. This land which God gave you, kill everyone in it. (OR, alternatively, the land that God gave us, we killed everyone in it.)
2. How to properly sacrifice animals to God.
3. What you are allowed to eat and not eat, who you are allowed to have sex with/not have sex with, what you are allowed to wear, etc. etc.
4. I am the Lord thy God, thou shalt have no other Gods before me. If you do, I will do terrible things to you.
Those four themes take up most of the pages of the OT.