What do Orthodox Christians believe about liturgy?
Biblically and historically true worship has consistently been liturgical. "Spontaneous" worship is an innovation of the last century or so.
Liturgical worship, written Prayers (the Psalms) and feast days were the norm throughout the history of Israel (see Exod. 23: 14-19; 24:1-289:4; etc.).
The worship of heaven is liturgical (Isa. 6:1-90; Heb. 8:1-3; Rev. 4).
The foundations of liturgical worship in the Church are apparent in the New Testament. The most oft-repeated prayer of the Church is there (Matt. 6:9-13). The words we say at baptism are there (Matt. 28:19). The words spoken at Holy Communion are there, with St. Paul repeating Jesus' words (I Cor. 11:23-26). Further, the believers in Acts 13:2, about 49 A.D., were seen in a liturgical service to the Lord: "As they ministered (Gk: leitourgouaton, our root word for liturgy!) to the Lord and fasted, the Holy Spirit said...." Note, too, in this passage that the Holy Spirit speaks to us during liturgical worship. Thus praise to God must never become dead form, but rather living worship, "in spirit and truth" (Jn. 4:23, 24).
Documents like the Didache (70 A.D.), the writings of St. Justin Martyr (150 A.D.) and Hipploytus (early 200s) all show the worship of the early Church was, without exception, liturgical.
Because of their disdain for Rome, some Protestant groups have reacted by dismissing liturgical worship (though everyone has patterned worship, "spontaneous" or not!) But the Bible and Church history are clear; liturgical worship is the norm for the people of God.