How we differ from other Christian traditions is something that you’ll encounter frequently during your catechumenate, and very likely from family members who belong to another tradition; the ability to explain how we differ then is a valuable tool for you to develop in terms of apologetics. There are historically two groups who have left the Orthodox Church, one of whom suffered a later, further schism. The first to leave were the Oriental Orthodox, who rejected the dogmatic definitions regarding the nature of Christ at the Council of Chalcedon in 451 AD. While in the 20th century dialogues occurred that revealed this to be mostly a linguistic misunderstanding, full communion has yet to be restored, so they continue to be known (to us) as monophysites, or, perhaps more kindly, non-Chalcedonians.
The next and most well-known to all of us is the Roman Catholic Church (including the Eastern Catholics/ Uniates, and other non-Latin rites) that broke away during the Great Schism of 1054. The causes began with disputes over papal authority and the inclusion of the filioque clause in the Nicene Creed (both of which we reject). Since then though there have been numerous Roman innovations that are incompatible with Orthodoxy, such as purgatory, the immaculate conception, papal infallibility, and so forth. While there are efforts between the current pope and the Ecumenical Patriarch of Constantinople to rejoin communion, it seems highly unlikely that ROCOR would accede to this, and so we continue to pray that Romans will repent and come home to Orthodoxy.
Protestantism presents an interesting and unique situation, as they didn’t really break from us, but rather from Rome; as such I’m usually quick to point out that most of their theological sticking points are at issue with Roman theology but not necessarily with ours. That said, their understanding of where their roots are, and how the Bible came to be compiled, the significance and centrality of Holy Communion, the need for confession, etc., make theological conversations complicated if not difficult. Due to distancing themselves from Roman theology, many protestants find difficulty accepting the ever-virgin status of the Theotokos, and do not share our views on infant baptism. Nonetheless, this is the group that forms the largest converts to Orthodoxy, largely due to a general lack of liturgy in most protestant worship.