I'm in the midst of reading the Old Testament right now (Phew! I made it through Leviticus). It is the first time I have read the Old Testament as a partially-actualized adult (not college student), and I am surprised. The God who I thought presided over the people of the Old Testament is different from who I thought he would be. So far this realization has caused me to rethink how I have studied the Bible in the past. I thought I would find God to be a rule enforcing dictator, and there are A LOT of rules in Leviticus, but when I finished reading Leviticus (and I read it, every stinking word), I was struck by how God gave those rules with the purpose of making the Children of Israel his. It is about being God's people vs. being a pagan people; being set aside for God. The "rules" aren't rules for rules sake. God is VERY about culture.
The point I am trying to make is that God changes the "rules." The rules are different in the Old Testament than they are the New--why? It seems to me that the purpose of the Bible is to show us that God strongly desires a close relationship with his people, not strongly desires "rule following" people. He is so strict in the Old Testament because the people are sooooo far from the people he desires them to be (Hello? Bestiality?). By the New Testament, they have cleaned up their act a bit, but they are so focussed on the rules that they forget about God.