Because of EO/OO issues, I've actually took the time to read most of Leo's writings available on CCEL. As I understand from EO's that a later Pope, Pope Gregory, rejected Papal primacy. If that's true, that's great. I always commended the two, Leo and Gregory, for two things that seem to go against present RC theology. One of Leo's sermons teaches that the Holy Spirit came and purified the Theotokos before Christ's conception. Gregory is quoted as saying "No bishop is above another." I usually use these two men, who were Popes of Rome even though not of my tradition to show that even them believed in things that seem to contradict the ideas of Immaculate Conception and Petrine Roman Papalism.
Nevertheless, the idea behind rejecting Pope Leo due to his alleged primacy powers seemed to be the emphasis and importance of accepting Leo's Tome without questioning it, because it was written in "the spirit of the authority of St. Peter."
I understand using St. Peter as an honor, but then you have issues like rejecting the idea of a "second Rome," the butting in on the affairs of other churches to ordain their bishops and patriarchs, the allowing of a condemned heretic by an imperial council to be brought back into communion without consulting the ecumenical Church, and simply his power magnified by the friendly relations with the emperor.