I would say that the sum of things that Catholics and mainstream Protestants have in common is much greater than the sum of things that separate us. In common, we have the Bible (with a small amount of disagreement about the canon), the Church Fathers, the first seven ecumenical councils, the creeds, broadly similar liturgies, and in many instances broadly similar understandings of apostolic succession and holy orders. Things that separate us would include disagreements about validity of apostolic succession and holy orders, papal authority, understanding the precise meaning of the Eucharist, purgatory, saints (to a limited extent), the Immaculate Conception, and some different views on divorce, remarriage, and birth control.