This goes back to the split between the Eastern church and the church in the West, a split between Eastern Orthodoxy and Roman Catholicism. If you go back even before that to the early church, you begin to see a difference between East and West, some of which is over the Trinitarian formula. The church in the West defines the Trinity as one substance in three persons. The church in the East tends to emphasize the three persons.
The other piece of the church in the East is the notion that is sometimes called *deification*. The saying goes something like this: Christ, who was God, became man so that we, who are men, can become God. That is different than our understanding of salvation, which involves union with Christ, being sealed with the Holy Spirit, and being brought into fellowship with the Trinity.
There is a verse in 2 Peter that speaks of being partakers of the divine nature, and that verse sometimes causes theologians in the East to set forth this idea that we become God. The Western church, in which Reformed theology came about as the church matured, had a clearer understanding of the goal of salvation.
If you are talking about the Reformed church and the Eastern church specifically, the Reformed church does not use icons in its worship and does not see icons as legitimate in the public worship of the church. In the Eastern church, however, icons are crucial to the church’s worship. They’re part of the church architecture. They’re part of the liturgy, and they’re woven into the practices of piety for the Eastern Orthodox.
The other distinction—and I want to be careful here but I think it’s a fair assessment—is that in many cultures where the Eastern church is dominant, it’s more of a cultural religion than a heartfelt, convictional religion, especially as the Eastern church has evolved over the centuries.
So, there are theological, liturgical, and cultural differences between the Eastern church and the Reformed faith.