Arguably, the Missale Romanum of 1962 serves as the most common introduction to the traditional liturgy. Although, by the time it was introduced, the Roman liturgy had already undergone decades of modernization. Traditionalist, by and large, accept the earlier decades of liturgical modernization as their own. Their praxis is frequently colored by local cultural customs of the early-to-mid twentieth century, such that one's country/culture of origin heavily influence what era one considers to be "traditional." They are, possibly to their own astonishment, the third stream in modern liturgy.
To understand the place Traditionalists occupy in modern liturgy, we have to sketch the process of modernization that formed their liturgy. It begins with possibly the most acclaimed pope in Traditionalist circles. Pius X's reform of the Roman Breviary put in motion the gears of modernization.
To begin with, Pius X's decision was based upon two dominant influences. First, there was the basic fact that the rubrics of the old Breviary made weekly recitation of all 150 psalms impossible, or, in all events, excessively rare. Second, I would argue, there was Pius X's perspective as the only pope in recent history to have actually been a pastor. The dramatic recasting of the Roman psalter not only did away with the common occurrence of priests only reciting a portion of the psalter each week, it arguably made the obligation to fulfill the office somewhat easier than it had been, via the reduction of the length of many hours. Yet, one cannot ignore that the dual motivations of reciting 150 psalms in a week and shortening the length of the office suppressed one of the most ancient psalter schemas in Christianity. The psalms were redistributed, sometimes even broken up, entirely new antiphons were composed in places, and ancient customs, such as the laudate psalms, were abolished. These changes led to complimentary changes to the Missale Romanum. Pius X's reform was so drastic that, in the North Eastern United States, many diocesesian clerics applied for an indult for fulfilling the office by reciting the Dominican Breviary - a useful way of preserving an older Western form of the office until the Dominicans revised their breviary to be in better harmony with the breviary of Pius X.
Subsequent changes to the Roman Breviary by Pius XII and John XXIII continued along the same trajectory, largely with the aim of abbreviating the length of the office and removing customs until there was a more basic form of the Roman Breviary. In fact, Pius XII, John XXIII, and the architects of the Liturgia Horarum saw themselves in the same continuum as Pius X's reform. One and all envisioned bringing to completion the reform begun under Pius X.
The modernization of the Roman liturgy continued in the Missale Romanum. Pius XII's reforms of Holy Week were conducted with the explicit intention of restoring the services to their hypothesized original form and function. The subsequent suppression of octaves, commemorations, feast days, liturgical classes and simplification of rubrics all congealed in the Missale Romanum of 1962, by which point the "Tridentine" liturgy was effectively a bridge to the more modernized Missale Romanum of 1970, in so far as certain major watermarks of the revised Roman Missal were by then present.
The position the Missale Romanum of 1962 has as the first exposure to pre-modern liturgy is largely due to incident and accident. It retains enough of the pre-modern liturgy to bridge the gap between modernized and non-modernized liturgy. In its simplifications and streamlined design, it anticipates the shaving off of ceremonial that accompanied the Novus Ordo. In its euchological content and calendar of saints, it functions as a living witness to an older tradition. Largely, though, it occupies its place because it was the Missal Lefebvre settled on for his society and, as a result, became the rallying point for all parties reacting against the imposition of Paul VI's Missal.
Rightly or wrongly, the Missale Romanum of 1962 has defined the Roman liturgy in terms of "pre-modernization." It is one of the two goal posts that set the match and define the boundaries of play. This causes a few quandaries.
In so far as their law of prayer is the liturgical books as the were in 1962, Traditionalists accept the principle of modernization. The dispute regards the degree to which modernization is acceptable. Tied into this question, there is an issue of ecclesiology. Traditionalist ecclesiology implicitly denies absolute infallibility to the Roman Pontiff. Even among those groups that have found communion with Rome, there is still a nagging issue over whether or not the public and definitive exercise of Petrine authority was correct in the case of the imposition of the new Missal, if not the convocation of the Second Vatican Council itself. Among those groups not in full communion with Rome, the denial is more explicit; plainly, the pope's authority is not absolute and there are things to which the pope can be held accountable.
To the degree that Traditionalist cause a crisis of ecclesiastical authority, and to the degree that the Missale Romanum of 1962 has come to be identified with pre-modern liturgy, the old liturgies of the Latin Church, Roman or otherwise, have been vested with something of a taboo quality. Anything that is not totally modern is suspect and discarded.
The crisis of ecclesiastical authority precedes a full ecclesiological crisis. Traditionalism often requires a subtle Gnosticism in that it functions off of the conviction that a large body of believers "have gotten it all wrong" and only they really understand the nature of things. It must be said, it takes some serious guts to tow a line like that. The better part of the Roman Church is subsequently determined to be, in some manner, deficient. This aspect of Traditionalism that seems most implausible due to the sheer numbers it would dismiss as irrelevant.
In the discussion of pre-modern liturgy, Traditionalism, fairly or un fairly, sets the goal post. However, the phenomenon is so loaded that, so long as it exists, it makes it nearly impossible to legitimately discuss the retrieval of liturgy once lost. Conversely, Traditionalism, whether it wants to or not, forces a re-examination of papal infallibility and supreme juridical authority. It does so by proposing there is a tangible limit to the authority of the pontiff. What this will mean in the long run, if anything, depends largely upon how Traditionalism determines its future course.
Next stop, Limbo.