Orthodoxy's dispute with Roman Catholicism's "Filioque," is probably the most substantive issue in dispute between the two churches, for two primary reasons.
1) The language of the Symbol of Faith was supported by scripture: "But when the Helper comes, whom I shall send to you from the Father, the Spirit of Truth who proceeds from the Father, He will testify of me," Gospel of St. John, 15: 26.
2) The Symbol of Faith (the Creed), was written and promulgated by the First and Second Ecumenical Synods, subsequently accepted by the clergy and faithful of the Undivided Church; and ratified by the subsequent Ecumenical Synod (Council). That rendered it doctrinal, infallible doctrine, a matter that must be believed. A Pope had those words emblazoned upon the walls of the Vatican in silver, so that those words would not be lost or changed. Yet, a subsequent Pope, and his followers, felt he had the authority to add language to those words of the Creed, centuries later. Ascribing that authority to one bishop of the church, not-with-standing his primacial authority, the Pope superseding scriptural based dogma of two Ecumenical Synods, was not accepted by Orthodoxy at the time, nor can it be acceptable today.
Then, to have the audacity to pronounce an "Anathema" upon the primal patriarch of the East for not accepting this papal innovation, raises the West's self-assumed authority, to be way beyond anything that Orthodoxy could even entertain for compromise resolution.