One of the key points the author makes is that the key disagreement between the two groups lay not in their views on the core Jewish theology such as the immanence of G-d, nor their belief in the importance of Kabbalah, but their evaluation of the ability of the regular Jews to properly understand the concepts. The Mithnagdim basically felt like only a few select, elite few could possibly understand the hidden mystical secrets after a lifetime of preparation. The Chassidim introduced a new concept, very different from Lurianic Kabbalah, in that the common people can automatically participate in the various tikkunim without even being aware that they are doing it, and that the mystical secrets are available to anyone who looks.
The Chassidim of Poland, Ukraine, Byelorussia were essentially trying to democratize the secrets of Kabbalah, to make it available to everyone. In some ways their logic made sense. Nadler quotes Meshulam Faivush of Zaborocz, Yosher Divrei Emeth:
I have heard from the holy mouth of R. Menahem Mendel [of Premyszlan], of blessed memory, that nistar is anything that a man is unable to communicate to his neighbor, such as the taste of a given food, which one cannot possibly relate to another who has never tasted of it. . . . So, too, the love and fear of the Creator, blessed be His name, cannot be communicated from one to the other—this, then, is called nistar.What is interesting to me is that somehow the Chassidim went from populists to monarchists, with the cults of the rebbes, the metaphors of the courts and royalty. From modernizers to the strong conservatives, where any new concept in either religion or lifestyle is viewed with mistrust and antipathy.
As for those who refer to the wisdom of the Kabbalah as nistar, what kind of nistar is that supposed to be? For is it not true that anyone who wishes to study the Kabbalah has the books ready before him. And if he is unable to understand the books, he is simply an ignoramus, and for such a person, the Gemara and the Tosafoth are also called nistar. Rather, the concept of nistar as referred to in the Zohar and the writings of the ARI is the ability to achieve devekuth.