As the dissenting voice on the "other forum", I felt duty bound to cast my vote here in favour of Koine and Slavonic.
My reasons are explained more fully on the other forum, but basically they are:
1) Koine is the Language in which the Scriptures were written, and Greek Orthodoxy is the only place where Koine is still a living language, and if in the Liturgy, Koine is replaced with modern Greek or other vernacular, there will be no living tradition left linking us to the language the Scriptures were written in, neither the New Testament nor the Septuagint.
2) Translations of the hymns and prayers of the Liturgy and Hours so often have different meanings. One hymn can be translated in three different weys, and the example I used was the Nativity Kontakion which, in one translation, had the Virgin come to the Cave to give birth to the "Transcendant One", in another translation, she gave birth to the "Uncircumscribed One", in another she gave birth to the "Almighty One". And while they are all true, they missed the original meaning of the Kontakion, which was that the Uncontainable God was Contained in a Cave.
3) The prayers and hymns of our Church are part of Holy Tradition, that is, they contain doctrinal elements and can be referred to when dogmatic disputes arise. If their original version is no longer a living Tradition, then we have lost this resource.
4) Given the current juristictional chaos which is the reality of the Orthodox Church in the Diaspora, each Synod would have to approve translations into the same vernacular language in each country for use in their juristiction. What happens when the translations differ?
5) Some people think that speaking Greek or Russian means that you completely understand the Koine and Church Slavonic which the Church uses, but this is not the case. We have to make an effort and learn the Liturgical languages too, so we are in the same boat. If the language becomes the vernacular, then Churches will be much more divided along ethnic and linguistic lines, with English speakers going to one church and Cantonese speakers going to another, and Portugese speakers going to another, and the groups will never worship together.
6) The Liturgy and Hours were never meant to be a way of missioning to those outside the Church. There is no reason why we can't spread the Gospel in vernacular without turning the Liturgy and the hours into a "recruitment drive" or "theatre" instead of the Sacred Mysteries and Prayer they are meant to be.