A few random thoughts:
First, to be Protestant is to be Christian and this implies that one believes in the Holy Trinity. Any group that denies the Trinity, even if that group came out of some Protestant tradition, cannot, as I understand it, be called Christian or Protestant.
Second, there is the sacrament (or ordinance) of Baptism. The commonality is that baptism is practiced by all Protestants (insofar as I am aware) even though there are disagreements about it. Does one baptize infants or must a person declare a “decision for Jesus” before baptism? Is baptism a sacrament or an ordinance? Does an ontological change take place in a person through baptism?
Third, there is a common rejection of the Catholic Church as the sole repository of the fullness of truth. For some, this means a virulent anti-Catholicism. For others, it means a recognition of Catholics as fellow Christians who simply disagree on some matters of doctrine. Also, there are those who continue to see the papacy as the anti-Christ while others recognize the Pope as an important Christian leader but do not ascribe to him the authority that the Catholic Church does.