Please explain more. My Biblical expertise is pretty limited. So which half is which?
There are two halves, the Old and New Testaments.
The Old is pre-Jesus. In it, god is jealous and wrathful. He makes a variety of cameo appearances, and generally terrorizes people when he does. This is also the part of the Bible where The Law is handed down -- rules for everything from diet to slavery to capital punishment.
The New is where Jesus comes in. Jesus, in a nutshell, teaches that everybody is a dirty rotten sinner, everybody needs forgiveness and mercy, and he's the only way to heaven. He challenges the entire socio-political structure of Israel, and the power players end up wanting him dead as a result. Jesus allows himself to be executed as a sacrifice for the sins of the world. This is important because in the Old, animals were sacrificed to cover up the sins of man, whereas in the New the blood of the only truly innocent man was sacrificed to wash away all sins for all time for all those who believe in the power of said sacrifice. This was Jesus replacing the old covenant between man and god.
Everything else in the New consists of what Jesus' Apostles did after that, and then the revelation of how the world will end.
I'm sure I missed a lot, but that's The 5-Minute Bible for Heretics Like Us.
Christians are supposed to stick to the New. Jesus replaced the covenant between man and god in the Old, and made it abundantly clear through his public acts that he was replacing the law, since the new paradigm he was bringing into the world was that man was to be saved by faith in him rather than obedience to the law.
Nevertheless, the Old is often cited by those with a political agenda, especially where homosexuality is concerned. This is where your perception about some laws being ignored and others revered comes in, because these same "Christians" don't hold to the dietary rules or really any of the other rules in the Old (except maybe the Commandments).
Does that clear it up?