The Petrine Office (specifically Papal Primacy, Supremacy and Infallibility) and Ecclesiology (monarchical/papal vs conciliar/patriarchal) are the foundational issue with Rome and the Orthodox… However, there are a whole HOST of issues which the Orthodox are at odds with the Catholic Church on.
First and foremost being the Filioque. As far as I can tell, the doctrine itself is no longer a serious issue… The issue is the Filioque being recited in the Creed and it being there in the first place.
The Orthodox have made it abundantly clear: so long as the Roman Church recites the Filioque, reunion is impossible. And there is no sign the Roman Church is going to drop a symbol of 1000 years of Latin patrimony… So we’re already at an impasse.
Then we have the issues of purgatory, indulgences, original sin, Immaculate Conception of Mary, unleavened bread in the Eucharist, Palamite vs scholastic theology and the associated issues of essence-energies and hesychasm, married Roman clergy / mandatory celibacy, divorce and remarriage and contraception, the nature of heaven and hell, use of icons vs statuary, fasting differences, calendar differences, issues with Latin terminology, issues with the Sacramental Character/indelible mark of Holy Orders, issues as to when the Consecration occurs (epiklesis vs words of institution), issues regarding post-VII reforms, issues regarding the Ordinary Form Roman Mass… And that’s all I can think of… Off the top of my head.
Add in the current fiasco between Constantinople, Kiev and Moscow and the fact that internal unity within Orthodoxy itself is crumbling, and you begin to see how foolish and misguidedly optimistic the notion of imminent reunion between the Orthodox and Catholic Churches truly is.