Two distinct methods stand at the foundation of our avodas Hashem: ahavas Hashem, and yiras Hashem. They are fundamental enough to be recognized, at least by allusion, in the very first word of the Torah. "Beraishis" can be read as "Bais" - "Raishis", or two beginnings of our service of G-d.
There is no question about the order in which they need be applied. The Zohar's introduction makes this explicit in calling yiras Hashem the "Pikuda Kadmaah", the first command, and ahavas Hashem the second.
This makes ample sense. The most essential task in avodas Hashem is uprooting the evil within us. Until a person purifies himself from his evil inclinations, it is impossible for him to attain love of Hashem or to attach himself to Him, which is the ultimate goal of the entirety of the Torah. At Shulchan Aruch's beginning, the Ramoh cites the words of the Rambam in Moreh Nevochim, "When a person takes to heart that the Great King HKBH, whose honor fills the universe, stands over him and observes his actions, he is immediately struck with His fear."
What holds true of the avodah of the individual also holds true in the unfolding of the sequence of generations. Before the word could arrive at the threshold of the Olam Hatikun (the world in the process of restoring itself to the lost brilliance of existence before the first sin), it had to focus on yirah. Only after the emplacement of a firm foundation of yiras Hashem could Avraham begin the process of contributing to the Olam Hatikun.
The task of securing that foundation fell to Noach, who served Hashem with the method of yirah. It is for this reason that the Torah underscores that Noach was righteous in his generations. He was the "Tzaddik Tamim," the perfect tzadik in completely fulfilling the particular need of the times in which he lived: the subjugation of evil through yirah. This made possible the next step of service through ahavah in the generations that followed. Noach is described as walking with Elokim - specifically the Name of Hashem that indicates the aspects of gevurah and yirah.
We understand as well the opinion in Chazal that views "in his generations" as a limitation of Noach's greatness. Why would the Torah go out of its way to speak "disparagingly" of Noach, as one opinion has it? The Torah's point, however, is not that Noach would have amounted to very little had he lived in the shadow of Avraham. Rather, it notes that in Avraham's day, the proper method of service of Hashem had already shifted to ahavah. (Note that when the Torah speaks of Avraham walking before G-d, it is the four-letter Name that is used, which is always linked to chesed, which is an outgrowth of ahavah. The difference between ahavah vs. yirah orientations explains why Avraham was successful in attracting converts to his cause, while Noach could not. Ahavah is an expansive, outward-reaching midah, making room for others. Yirah focuses inwards.) Once a generation has reached a level that it can occupy itself with ahavah, focusing on yirah would have been a step down. Practicing yirah in his own time, though, was precisely what was needed, and what Noach was supposed to do. Had he tried his hand at an ahavah-oriented service, he would not have discharged his responsibility.
We can further explain Noach's choice of yirah by noting that he lived in a thoroughly corrupt society. When times are enveloped in a miasma of tumah, there is no choice but to serve Hashem through yirah. We recall the teaching of the Divrei Shmuel [grandson of the Saba Kadisha of Slonim], commenting on Yaakov's words of surprise when Yosef's two sons were brought to him. "Mi Aileh"- who are these, he asked. In gematria, "Mi Aileh" equals Elokim, the Name of gevurah and yirah. Yaakov's question referred to the manner in which the two children served Hashem, choosing yirah exclusively. Yosef responded, "They are my children, whom G-d gave to me in this," meaning in a morally depraved country such as this, necessitating that they focus exclusively on yirah. Yirah is the only strategy for times beset with gargantuan tumah.
The lesson applies directly to the individual. When the yetzer hora rises up suddenly and appears indomitable, only yirah will offer protection. (At the climax of the Akeidah, the angel tells Avraham, "Now I know that you fear Hashem." Wasn't Avraham's special midah ahavah, and not yirah? To successfully navigate the most difficult test of his life, however, ahavah would not have sufficed. Avraham had to move away from his midah of choice, and use yirah to defeat his yetzer hora.)
Yet another factor demanded the different kinds of service of Noach and Avraham.
In times of overwhelming tumah, such as the pre-Tikun times of Noach, it is necessary for good people to draw back, to seal themselves up in isolation. At times that kedushah is ascendant and more available, it is proper for people to fan out and spread the light of Hashem far and wide. Avraham realized this, and responded. When Chazal opine that the Torah speaks "disparagingly", the target is not Noach himself , but his generation. It was so steeped in tumah, that Noach had to serve Hashem through yirah alone.
The Torah is eternal. The parshah of Noach reminds us that each generation has its own special method of serving Hashem. This applies to individuals as well. Each individual is charged with a unique avodah, dictated by his individual strengths and leanings, as well as the exigencies of his times. The ultimate foundation, however, remains yiras Hashem.