R. Michael Broyde's Review in tradition is generating a lot of heat over at Hirhurim. My comments below, that I cross-posted there, earned their fair share of criticism. (Though, I must say, I find it sort of odd that I'm being painted as an extremist.)
In any event, in order to clarify my remarks here, this is what I wrote there:
Regarding my basic point as to why Kabbalat ha-Mitzvot is under discussed among Rishonim and pre-Modern Ahronim:
I described the sources in light of accepted axioms of the History of Law. My point was this: Things that go without saying usually do but are no less binding for that. The Gemora in Yevamos doesn't require that the convert undergo a course in Jewish Theology (though the Rambam does, but as the late Prof. Twersky noted, that's an innovation of Maimonides). Yet, can anyone imagine a Jewish conversion wherein the convert does not accept Ol Malkhut Shamayim? Obviously not. Law exists in a socio-religious context. The context determines what gets written down and what doesn't. Therefore, trying to pry all sorts of tortured nuances out of Rishonim on this issue is a testimony to the fact NOT that Qabbalat ha-Mitzvot was not required but that it was so obvious as to not require mention.
Regarding the observation that the first rabbinic authority to really address the subject head on was the Tosafists in the twelfth century:
Why do Tosafos discuss it? Very simple. As the revivers of Talmudic dialectic and the advancers of what is called the 'critico-conceptual' approach to Torah study, Tosfos analyzed the component parts of every aspect of Halakha. Kashrus, Sheckhita, To'en ve-Nitan, Kodoshim, Niddah etc etc etc. So it was only natural that when engaging the various sugyos on gerus, they did the same.
There is, in my opinion, no reason to claim that Tosfos 'invented' the need for Kabbalas ha-Mitzvos. In fact, if there is a contradiction between the various Talmudic passages on the need for accepting the commandments, why didn't Tosfos note it. Yet, as far as I know, nowhere in the entire corpus of Tosafist Halakha is there any mention of such a contradiction. Does it make any sense to anyone that Tosfos would notice contradictions between Gemoras and Midrashim, but pass in silence over an glating contradiction between a Mishna, a Gemora in Bekhoros and a Gemora in Yevamos?
Again, I agree with R. Broyde (and I think a strong case can be made that some of Reb Moshe Feinstein's responsa would support this) that a critical distinction should be made between Kabbalat ha-Mitzvot and Shemirat ha-Mitzvot. Reb Chaim Ozer leaves open the option of accepting converts who will define themselves as Sinners.
Serious food for thought.