A lot will be written but I wanted to share my perspective [with no claims of being objective truth... I am biased because I liked him. I am also biased because I never went to YU or any college for that matter and chose to raise a family with a more Yeshivish orientation. But who ISN'T biased?].
For most of the 20th century, Modern Orthodoxy was not really an ideology. It was just about being lax in religious observance. Most Jews discarded observance altogether while others decided to keep some of the tradition such as marrying within the faith and keeping kosher [at least at home] but also imbibing in non-halachic behaviors such as mixed dancing [sponsored by the shuls!!]. The latter was basically Modern Orthodoxy. Learning Torah was important but only enough to require basic knowledge of a few halachos, famous parts of the Chumash and basic hashkafa [One G-d who is involved in our lives, we don't believe in Yoshke, Israel is important, we are the Chosen People, shuls should have a hot kiddush every week after davening, true cheirus on Pesach can only be felt in a hotel etc.]. Getting into Harvard would be preferred any day over getting into Brisk or Chevron. On the contrary - it was considered a tragedy when a child chose a life of intense commitment and religious passion. That was it. Just a parve, lukewarm Judaism. Better than Conservative and down but not authentic, passionate Yiddishkeit.
Then came Rabbi Lamm, especially in his classic book "Torah U'madda" and argued that to be a "Centrist Orthodox" Jew is JUST AS AUTHENTIC as the "right wing" or Charedi variety. Knowledge of Hashem comes through knowledge of his world and thus secular education is part and parcel of Avodas Hashem [it helped to have the Rambam and many other greats on his side]. It is "Torah U'madda" and not "Torah U'movies" or "Torah but I'm not shomer negiya and don't bentch during the week".
This was a revolution. I know that this made a deep impression on me. Being more extreme, radical or close minded doesn't necessarily make one more religious. As he put it "We are passionately moderate and not moderately passionate". His vision was imbuing even the most secular aspects of life with Kedusha. Being a good Jew doesn't require one to spend all of one's time on the Beis Medrash, certainly not for the masses. בכל דרכיך דעהו. Know Hashem in everything you do. Be part of the world and elevate it. A majestic vision. As he put it - We are a light unto the nations and not a Neturei Karta unto the nations.
Love it.
So in theory it is hard to argue. His arguments were cogent and well sourced.
But alas - it was not a fool proof argument. Going to Harvard involves much more than just studying and appreciating the grandeur of Hashem's creation. There are the tzniyus issues that exist in every university and then there are the heretical ideas that often drive kids away from observance completely [I wish that I could say that in YU the students weren't taught kefirah but from what I understand not every course taught is consistent with what we want our children to believe and not every teacher is a great Yarei Shomayim וד"ל]. One can also question whether the motivation of many Modern Orthodox people is pure and about Avodas Hashem. They watch the Super Bowl to somehow be מתקן עולם במלכות ש-די? I'm not convinced. After 15 years of MO education how much do our children know? Why do most kids who get serious about Torah observance only do so when they come to Israel for the year? And how many lose it afterwards? And how many people fight VALIANTLY in order to prevent their child from spending additional time studying Torah in Yeshiva or seminary? A lot of the laxity regarding halachic observance, lack of Yiras Shomayim and full commitment still pervade. But large positive strides have been made in the last few decades and particularly in the last few years. When I was a child growing up in a MO community, I knew almost no adult who was kove'a itim and almost no married women who covered their hair. Weekday minyanim were not well attended. Today these and so many other things have improved drastically.
What we owe Rabbi Lamm a debt of gratitude for is that he presented the vision and showed the validity of a life where one is engaged in the outside world but all in the framework of a fuller, more expansive Avodas Hashem. As Maran Rosh Ha-yeshiva put it in a famous letter to a Talmid who felt inadequate for working for a living [printed in the אגרות פחד יצחק] - You are not living a "double life" but a "broad life". Everything is subsumed under the umbrella of Yiras Shomayim, Ahavas Hashem and Avodas Hashem. A harmonious synthesis.
Did anybody else set forth such a clear vision for Modern Orthodoxy in our times? Rav Aharon Lichtenstein and ... I can't think of anybody else [but it is 2 am so maybe it is because of fatigue]. If there are more [and there probably are] they are few and rare.
Does one have to agree with everything Rabbi Lamm said and taught? No. Does one have to think that YU is perfect? No. Did he make mistakes in his life? We all do. [After his death some people were quick to fulfill the Maamar Chazal (זריזין מקדימין לעבירות) that those who speak Lashon Hara about a Talmid Chochom after he passes will fall into gehenom and spread disparaging things about him around the Internet. Truthful or not, it is definitely disgusting]. Was his relationship with the Yeshiva world at times strained? Definitely. But to his credit he tried to always be respectful [which is hard when one is attacked so fiercely as he was]. We should all appreciate the contribution he made to Klal Yisrael even if we don't sign on to everything he said and did. And there is so much more that I haven't mentioned in this appreciation and will definitely be written and spoken by others.
יהי זכרו ברוך!!!