Another important "Catholic Principle": Communio
In fact, I wouldn't call it a "principle", but rather an "aspect" of being "catholic". Acroamaticus, like many non-Catholics trying to be "small-c catholic", concentrates on dogmatic characteristics. But the Creeds remind us that there is something else in view: that Church which is Holy, Catholic and Apostolic is also One. This means the ecclesiological question cannot be dodged.
Catholic ecclesiology speaks of the One-ness of the Church in terms of an "ecclesiology of communion" (as does the Orthodox Church - this is something we share with them, thus proving that it is indeed a "catholic principle"). My assertion that there is no meaning to the term "catholic" if it does not include "communion with the Bishop of Rome" may give rise to argumentation on the "bishop of Rome" side of the equation, but noone can deny the necessity of the "communion" aspect if we are seeking to understand what we mean when we say "catholic". For indeed, how can the church be "catholic" in the sense of "according to the whole" or "universal" without entering into the question of how local churches are united in one visible communion?
One of the chief reasons that I became Catholic is because, despite my deep Lutheran spirituality, I could no longer continue in a church body in which the universality of communion of the one, holy, catholic and apostolic Church was given no concrete expression whatsoever. That, and the fact that I became equally convinced of the other end of the equation, the necessity for this universal communion to be united in communion with the Successor of the one to whom Christ said "I give you the keys of the Kingdom" and "On this Rock I will build by Church".