That being said, I just found a great article by Avery Cardinal Dulles on Deism in First Things. He explains that for all its claims to reason, universalism and objectivity, its tenents were inescapably formulated in Christian terms.
Although deism portrayed itself as a pure product of unaided reason, it was not what it claimed to be. Its basic tenets concerning God, the virtuous life, and rewards beyond the grave were in fact derived from Christianity, the faith in which the deists themselves had been reared. It is doubtful whether anyone who had not been brought up in a biblical religion could embrace the tenets of deism.
Moreover, the Deist God--completely detached from the world--was a useless God. Dulles writes, "The pupils of the deists, carrying the critique of religion one stage further, questioned the existence of this idle Supreme Being. Thus deism came to be a halfway house on the road to atheism. Toland drifted gradually from deism into pantheism."
He goes on to sketch out some of Deism's principle philosophical flaws.
"They lacked the metaphysical principles needed to build a viable natural theology. Empiricists like Locke and rationalists like Newton lacked the rich ontology of Thomas Aquinas and the medieval schoolmen. Their epistemology was a shallow empiricism and their cosmology a universalized physics, both of which crumbled when faced with the penetrating critiques of David Hume and Immanuel Kant.
Additionally, the deist system suffered from some internal tensions. If there is an omnipotent God, capable of designing the entire universe and launching it into existence, it seems strange to hold that this God cannot intervene in the world He had made or derogate from the laws He had established. He might have good reasons for bestowing some added benefits not contained in the work of creation. American deists such as Jefferson and Franklin did not rule out all divine intervention. They were convinced that God punished evil and rewarded virtue both in this life and in the next. They also encouraged prayer in ways that seemed inconsistent with deism in its pure form.
If God was infinite in being, moreover, it was unreasonable to reject the
notion of mystery. It would seem quite natural to suppose that there are depths
of the divine being surpassing all that could be inferred from the created
world. We cannot know what is going on in the minds of our fellow human beings
unless they manifest it by word or deed. How much less, then, could we grasp the
thoughts of God unless He were to disclose them to us by revelation?..."
Dulles also discusses the critical reconstructions of Jesus' life offered by the Deists.
Finally, the deist reconstruction of the historical Jesus lacked any serious
foundation in biblical research. Jefferson claimed that it was “obvious and
easy” to distinguish the authentic words of Jesus from those attributed to him
by later Christians. In his view they were “as easily distinguishable as
diamonds in a dunghill.” But even the most confident members of the Jesus
Seminar today would make no such claim. Jefferson fell into the common error of
simply projecting onto Jesus the moral ideals of his age.