Not a small sticking point, specially considering the innovative elevations that Latins have given to the office since the split.
A good start:
While still a brilliant theologian at the university, he endeared himself to the separated Eastern Orthodox with his famed “Ratzinger Formula.” In Graz (1976), the Roman Church dogmatician shocked the ecumenical world by declaring that “what was possible during a whole millennium can Christianly not be impossible today.” Consequently, “on the doctrine of the primacy (of the papacy), Rome must not require more from the East than what was formulated and lived out during the first millennium”–that is, prior to the 1054 Great Schism.
Ratzinger later clarified that his 1976 statement was not meant as a mere chronological return, but as a mutual commitment to confess the essential doctrinal consensus that had emerged as the ecclesial heritage of the first seven ecumenical councils of the undivided early church (through II Nicaea, 787).
http://www.carthage.edu/augustine/index.php?page_id=10/