Genesis 6:6 reports of God's inner state: "vayinahem" on account of God having created humanity upon the earth. Commentators suggest this word to mean that God "regretted" creating humanity or alternatively (and strangely) that God was "comforted" by the fact that God had created humanity upon the earth (as opposed to in the heavens). See, e.g., Sanhedrin 108a. In either case, this "vayinahem" is quite a turnabout from the use of the same root a few verses earlier in Genesis 5:29, in which Lemech declares that his son Noach would offer comfort ("yinahamenu"). The n-h-m of 6:6 presages utter destruction -- not comfort or consolation.
Reference to Exodus 32:14 -- another instance of "vayinahem hashem" -- yields interesting insights about human agency in the world. God resolves to utterly destroy the Israelite people and to create a great nation, starting from scratch with Moshe alone. Sounds familiar so far, no? This time, however, Moshe petitions God for mercy. The result is: "And God regretted (vayinahem) the evil that He had resolved to visit upon His people."
This is an exquisite parallel opposition. In the case of Noach, God's inner regret augurs utter destruction of humanity. In the case of Moshe, God's inner regret augurs the merciful sparing of the people. The latter case is brought about as a consequence of Moshe's heroic intervention on behalf of the people.
If you spend time comparing these two episodes, you'll find many other linguistic parallels and wordplays. One of my favorites is when God tells Moshe (before Moshe even opens his mouth in protest), "Leave Me alone" (haniha lee). Note the recurrence of the root letters in Noach's name (n-h). God wants Moshe to leave Him alone, just as Noach left him alone to carry out his destructive plans. But this time, it is different.
In prevailing upon God to change the annihilative plan, Moshe has exceeded even Avraham's greatness when he pled with God to spare Sodom. Unlike Avraham, Moshe's plea prevails. I'd suggest that this is what is meant by those commentators who opine that Noach was righteous only "in his own generation," but that had he been in Avraham's (or Moshe's) generation he would not have been considered righteous. See, e.g., Rashi on Genesis 6:9. God is in search of humanity's genuine partnership -- a partnership that is fully realized in the person of Moshe, who challenges God's designs. Noach offered the righteousness of obedience only.
I'd further suggest that's the significance of the statement in Avot d'Rabbi Natan that Noach was born circumcised. Circumcision can be understood to represent the need for human agency in creation. (Apologies for the exclusively male paradigm; and I think the idea can easily be reworked in a feminist light.) Avraham earned the quality of wholeness ("tamim") when he underwent circumcision; Noach, having been born "tamim," lacked the insight that was borne of Avraham's experience in the world.
Back to Genesis 6:6 - "vayinahem": Rashi suggests in a "davar acher" that the word means that God's thoughts were transformed from the quality of mercy through which God created the world to the quality of judgment through which God destroyed the world. Avraham petitioned God's quality of judgment on behalf of Sodom: "Shall the judge of the whole world not do justice?" Avraham's appeal did not prevail. Moshe's petition, on the other hand, succeeded in moving God from the place of judgment back to the place of mercy necessary to sustain the world. Vayinahem hashem.
