The question of the use of Church Slavonic (henceforth CS) vs. English in the Liturgy is a very important one. It reflects far deeper theological and psychological issues than might appear on the surface.
The real problem has to do with social psychology and cultural cycles. The feelings, motivations and aspirations of people living in the early, creative period of a culture are very different from those living at that culture's terminal phase. The former tend to be eager, zealous and hopeful, the latter despondent, cynical and nihilistic. The first Slavonic translations of the Holy Scriptures in AD 862-63 by SS. Cyril (then Constantine) and Methodius were made at the dawn of Christian culture, at a time when faith was much more powerful than it is today. Theological issues were very real and complex and bore a direct and urgent relation to peoples' perception of the world and the detailed structuring of their lived, day-to-day realities. These issues reflected a deep concern with the appropriate formation and training of the soul, and with subtle and complex ascetical and spiritual values which are scarcely comprehensible today, even among devout and practising Orthodox. A concern with such issues is a hallmark of both 'existential' and 'moral' dimensions of general intelligence, dimensions which modern psychology has entirely lost sight of, and yet which once formed the basis of what Fr. John Meyendorff (of blessèd memory) has called "Byzantine Rational Ascetic Theology" ("An Introduction to Patristic Theology - Введение во святоотеческое богословие" (2001)) or what Giambattista Vico (in his New Science (1744)) called "a Rational Civic Theology of Divine Providence'. The schism between the ancient liturgical languages (Hebrew, Greek, Latin and Slavonic) and modern vernaculars are the reflection of a deeper schism or disjunction between genesis and validity, between the transcendent and the mundane, or between the divine and the secular.
It's a question of cultural evolution. As cultures evolve in response to social and environmental challenges, semantic and linguistic drift over the course of time frequently bring about a dissociation between a concept, feeling or object which is experienced and identified (the signified) and the word which was formerly used to describe it (the signifier). The ancient liturgical and scriptural languages often used allegory, analogy or metaphor to describe spiritual or psychological states that the modern person can no longer identify with since the conceptual signifiers which once pointed to them no longer exist in the modern vernaculars - the patois of what has become the highly mechanised, sensualised and terminal phase of our culture. A typically 'patristic' statement such as 'the fires of apathy (in the original meaning of Greek απάθεια) illumine the eyes of the heart' is incomprehensible to a 'modern', which is tragic, since this deep and powerful statement (from the writings of St. Isaac of Nineveh) would require several rather fat modern 'self-help' manuals to explain it. What do 'poverty' (CS: нестяжáние), 'chastity' (CS: целомýдрие) or 'humility' (CS: смиреномýдрия) mean for 'modern' people? How do the CS terms convey the very essence of those experiential states? What exactly is the difference between целомýдрие ('chastity') and чистотà ('purity')? Yes, there are 'modern' words for these expressions, but the modern equivalents usually convey somewhat pejorative meanings, even if subconsciously.
The aggressive disparagement of CS (especially in the OCA) is also a consequence of a creeping corporatisation and Protestantisation (sorry for these clumsy words) of Orthodox Christian parishes, especially in the US, in which the priest (as has been stated above) has no greater status than that of an employee of the Parish Board of Directors (oops, sorry!) Council. The duty and responsibility of a parish council is to discuss and resolve issues relating to parish finances and general business, true, but certainly not to dictate liturgical practice to the priest, whose authority in these matters derives directly from the bishop or Metropolitan. What has God to do with Mammon (Matt. 6:24)? And In any case, what will be the final fate of a civilisation which allows itself to be governed by 'merchants'?
Yes indeed, we may well take note that the most agressive stance against the ancient liturgical languages is taken by the 'baby boomers' - who, after all, were the dominant age cohort over the last half of the 20th century and who catalysed the breakdown of individual and social morality, the disintegration of the family and the rise of corporate culture (the present writer admits to being a member of this cohort - it's his personal equivalent of 2 Cor.12;7)). This generation's time, however, is finally passing, and when they have gone, the faithful will hopefully be able to rescue at least some of the babies that were thrown out with the bath water.
In no way, let it be understood, would we even consider abandoning use of the vernacular in Church. Understanding of the Word of God in the language of the people is absolutely essential. We would only advise that conservation measures be taken in those parishes where the appropriate resources, both human and material, are readily available. Two things might be done in English-speaking parishes. First, build a higher level of spiritual, historical and linguistic education, including study of the Holy Scriptures and of the Philokalia (in English of course! and of the basics of the ancient liturgical languages etc. using, in the case of CS, accessible and relevant grammars such as that of Fr. Alypy Gamanovich of Holy Trinity Monastery, Jordanville (2001) or others (see a good resource list at puluka.com) - i.e stick to practical, liturgical CS, not the Old CS primers written for linguistics experts. The Orthodox Church in Poland for example has done a really fine job in creating CS materials for young children. Second, serve at least one liturgy per month in CS - which would be preferable (IMHO) to mixing languages in the course of a single liturgy, a strategy which often pleases no one).
As Christians, we should be aware that we are now living in a culture that has come full circle and returned to its point of origin - the culture to which it is affiliated, i.e. the Roman Empire. The historical parallels are striking, and the final tragic act will be similar. This historical awareness should be part of our Christian education. Such awareness would help us cope with the society we are living in and neither be seduced by its distractions nor corrupted by its false teachings.
It is to be hoped that the termination of this historical cycle will be succeeded either by the Final Judgement or by an affiliated culture in which the ancient spiritual and moral values will once again come to form the basis of a new Christian anthropology.