IT'S GON RAIN!

[Genesis 6:1-4]
This is a really strange passage, but it's interesting. Taken at face value, it sounds here like angels and demons were coming down to Earth and getting it on with humans. The offspring of these encounters were giants and the "mighty men who were of old." I heard an interesting theory that these mutant offspring of angel and man were the inspiration for many fables such as Hercules, Achilles, Gilgamesh, and other such tales. The argument being that many cultures have developed similar stories, so it could be that they were based on some common memories of super humans like these. It's not airtight, as the idea of "a guy, who's a normal guy, except he's REALLY strong!" doesn't exactly take a large brainstorming session and could easily have been developed in a vacuum. Still, it's kind of a cool thought.
Interesting history musings aside, this was part of what made God unhappy with how things were going, and He decided He didn't have enough patience to keep letting these people live 800 years. For this and other reasons, God started reducing the average lifespan to 120 (which is still pretty nice by today's standards.)  God doesn't like inappropriate relationship with the spiritual realm. Whether it's having sex with an angel, witchcraft, or even manipulative prayer rituals, God has cut lifespans to 1/8th of what they used to be for doing things like that in the past, so it's probably a good idea not to do it.

[5-12]
Upon further reflection, He realized "Actually, 120 years is still too long deal with these guys. Your lifespan is now 'until I get this flood together.'"
So things got crappy to the point where God decided He was going to wipe everything out and that the whole thing was a mistake. The only thing saving the world from complete destruction was that God thought Noah was a pretty cool guy, so He decided not to kill Noah.
Sensationalist fear-mongering pastors whose churches you should leave and over-zealous youth leaders whom you should fire use this passage to preach "GOD IS NOT AFRAID TO DESTROY YOU! SO DO WHAT I SAY AND GIVE MY CHURCH MONEY/ DON'T KISS YOUR GIRLFRIEND." The focus being on "evil people get squished, and good people get on the boat." This does seem to lead nicely from the passage, and, if we were still under the old Covenant, would be a fantastic message. In the New Testament reality we live in now, however, what you should take from this passage is "I'm so glad God sees Jesus' righteousness when He looks at me, and not my own, because otherwise I would totally deserve a flood right now." Noah found grace in the sight of God because he defied everything about human nature and managed to live in a way that was relatively pure when compared to the other people alive at the time. Now, all it takes is the blood of Jesus to find Grace in the eyes of God.
Unfortunately for fear-mongering pastors and youth leaders, this means we'll have to get people to act right and stay pure by teaching them that God is worthy of our respect and that relationships are a beautiful covenant and not a toy. This upsets people because you don't get a cult of brainless sheep out of it and it's not as fun as screaming.

[13-22]
There are further lessons here though. After God decides He's going to spare Noah, He gives him a set of instructions on how to build a boat that's going to keep him alive while God's wrecking everything else. He gave dimensions for the boat and instructions on what to take with him. Now it was up to Noah to fulfill these instructions. Presumably, if Noah had fudged on some details, or not taken enough food, or not taken enough animals, he either would have died with everyone else or the repopulation effort would have gone horribly wrong. Noah found grace, but still had a responsibility to make sure he got all the benefits of it.
Thanks to Jesus, all Christians have found grace with God. That does not mean that we are free of responsibilities or instructions. Life still has plenty of dos and do-nots that we can royally screw ourselves up by not observing. There are still things God likes does not like, there is still right and wrong, and there is still wisdom and foolishness. The difference now is that the consequences have shifted from punishment to discipline. The negative consequences from these actions now are not God saying "Well, you messed up. Guess it's time for a smiting" but instead happen so that we will learn and grow. We're not "paying a price" for our sins anymore - Jesus paid all of that price and if you think you can suffer in any way that will help pay for it, you think far too highly of yourself - instead we are just suffering the consequences of why we were told not to do that in the first place.