A whole slew of things, and depending on which commentaries one consults, different themes get emphasized. The one that comes to me off the top is this: From Adam and Eve to Cain and Abel to Noah and the flood, one of the major threads is illustrating the concessions to human nature that must be made in making any (lasting) relationship (not just with God) feasible. The reader is made to realize that human nature will give rise to all sorts of unpleasantness, but despite how aggravating, destructive, petty and selfish we can be to one another, the relationships are worth it - thus God's vow not to destroy civilization again, because "man's heart's desire is evil from his youth," not in spite of that.
Another is to contrast Noah's behavior with that of Abraham. Noah was righteous, and did as God commanded, but nothing more; whereas Abraham, when he discovered what was in store for irredeemably selfish, evil Sodom, nevertheless confronted God, seeking to clarify whether in fact destroying the city would serve the ends of justice. Jewish tradition is ambivalent on Noah's level of piety given that perceived lack of effort: "...Noah was a righteous man; he was innocent in his generations..." There are two ways of interpreting that last clause - either that Noah was so righteous as to be considered so in any generation, or that in his generation he was really something, but if compared him to Abraham he was nothing special - specifically because Abraham interceded for the evil people of Sodom. When people bring upon themselves whatever misfortune they deserve, what's our response? And how will we live with that response afterwards?