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This is similar to this response I made to the question, 'Do Evangelical leaders (televangelists, megachurch pastors, etcβ¦) not consider themselves Protestants?'. But, essentially the primary doctrine that set Protestantism apart, and was a primary cause of the reformation to begin with is Justification By Faith: that salvation comes entirely from Christ's atoning work on the cross.
Without turning this into the wikipedia article on protestantism, there are a number of other doctrines that have come to dominate the thinking of most modern protestants including, the priesthood of all believers (which is why protestants call their clerics pastors instead of priests), and the five solas, including sola scriptura ("by Scripture alone").