If Mormons aren't Protestants, then how are other Restorationist traditions Protestant? or even Methodists? The history of Methodism is distinct from attempts to "reform" Catholicism, after all
My not-so-intellectual understanding of the distinction (as has been addressed already) is that it is more about from which denominations you descend rather than your particular beliefs. All Protestant denominations descend from Luther "protesting" the Catholic Church without any inherent desire to leave and/or deny the institution as at least legitimate in theory. Each new Protestant faith is building upon further critiques of the Catholic Church, and while they are all now separate denominations (obviously), I think they can all clearly trace their lineages up the family tree. That is why even some of the drastically different Pentecostal churches (i.e., handling snakes and stuff) are considered Protestant without any arguments by most people. I might be ignorant, but from what tradition does Mormonism claim to descend? To add an entirely new prophet is hardly a small critique of an existing faith.
So are Mormons unique among Restorationists only in that what they believe about continuous revelation is just a *bit* too far gone? That seems a very tenuous place to try to erect some kind of litmus test
Restorationism does not consider itself a "descendent" of any type of doctrinal Protestantism - the Churches of Christ, Disciples, Adventists, Christadelphians, Pentecostals and Mormons are alike in seeing themselves as a
restored apostolic church, not some incremental reform of Catholicism. There is not a clear denominational lineage from which you can trace the Restoration movement back to Luther or the other Protestant reformers: this is the pioneer religion of the American frontier.