The gist of this chapter is that mankind got evil, and they got evil quickly. We sure didn't need long to "evolve" in our sin natures. Barely ten generations of recorded history in, and our creator God was sick at heart. He was ready to take it all back to the drawing board.
I take two things from this chapter. First of all, history repeats itself. Today's society does not differ much from Noah's. People "take them" intimate partners "of all they choose." Either through divorce or outright immorality, people defile themselves within themselves without much thought. In addition, man had become increasingly violent, which is a natural result of the strong presence of sin and a strong absence of the Spirit.
Second, though you may be alone in doing right, it is still important to do that right. God notices, and it thrills His heart! The earth's entire population was in sin, yet Noah "walked with God." Those are the same words used to describe Enoch, who God saw fit to translate without the experience of death. Noah's obedience saved him, and it also effected the salvation of his loved ones!
1) on Violence--Given it's own post heading.
2) On the "Sons of God." Nothing theological here. I just found it interesting. I remember hearing in a Bible class, that the "sons of God" might possibly have been the angels that joined Satan in his rebellion and were then cast from heaven. The result of their unions with human women produced children of remarkable size, strength, ability, and reputation. I can't help wondering if it was offspring of this nature that gave rise to the Greek and Roman stories of gods and demi-gods.
3) Specific Instruction--When God wants something done a specific way, he takes the time to give the details! On the other side of that coin, if God takes time to give us the details in regards to something, we had better pay attention and obey to the letter. Just as I love my children as they are, but expect them to obey me when I instruct them; my Heavenly Father loves me as I am, but expects my obedience. Partial or half-hearted obedience still falls below par!