As the feast of the Immaculate Conception approaches an opportunity presents itself to demonstrate that, while the Roman liturgy on the whole was in a slide since the Council of Trent (owing to centralization, I actually like the 1570 Missal's balance quite a bit) there were spots of legitimate and true development. One was the feast of St. Joseph Patron of the Universal Church instituted by Pius IX. Another favorite example of mine was also instituted by the same Pius IX, the feast of the Immaculate Conception.
The celebration of the Conception of the Blessed Virgin Mary is well attested in older manuscripts of Latin rite liturgies. One edition of the Sarum Missal shows a variation of the Gaudeamus omnes introit and Mass used in that diocese. The Roman practice is more difficult to ascertain. Several scanned copies I possess of the 1474 Missale Romanum list the "Conception" of Our Lady for December 8th, but gives no actual Mass in either the temporal or sanctoral cycles. Alas, one copy does indeed give an actual Mass for that day, with an introit based on 3:11 of the Canticles of Solomon, Egredimini: "Go forth and see your queen, daughters of Sion...." The Gospel is from St. Luke, not that of the visitation, but that of the Marian votive Masses. For whatever reason St. Pius V more or less abolished this text in favor of the familiar formulary Mass Salve Sancta Parens which we often hear on Saturdays. Indeed it seems to be, more or less, the same Mass as on the Nativity of the Blessed Virgin, September 8th, with a few words altered for the difference. Perhaps the saintly pontiff was overcome with a little Dominican prejudice against the Scotistic argument in favor of the Immaculate Conception? Having no manuscript scans of the Roman Breviary I cannot comment on its contents for December 8th.
St. Pius V's decision to employ a formulary Mass rather than a proper one demonstrates a unique feature of the Roman rite, the "Common" Masses for various types of feasts (Virgin martyrs, abbots, confessor bishops etc) when the local rites would have a unique Mass. Gregory DiPippo of New Liturgical Movement is in the midst of an interesting series on weekday lectionaries in the Latin rite which covers this topic more than I will today. Indeed one must ask if the actual parishes of Rome, before the Franciscans persuaded Pope Nicholas to impose the Curia's books on everybody in the City, had their own readings for certain days such as December 8th. Again, as has been said elsewhere on this blog, the Curial books are far less elaborate than most diocesan rites and seem to be abbreviated rites for use by people who have limited prayer time throughout the day. It seems possible that having so many Commons and formulary Masses may well have been one uniquely Curial trait that became a general Roman trait.
This Salve Sancta Parens Mass lasted until Pius IX defined the Immaculate Conception, necessitating, it seems, a more unique Mass for the occasion. The result, the Gaudens gaudebo Mass of today, is on the whole an improvement and development. The introit is a little odd to me, I guess one may say that Our Lady was filled with joy of Our Lord's making. The collect is a recovery from the pre-Tridentine Roman books, discarded in 1570: "O God, Who by the Immaculate Conception of the Virgin, prepared a worthy dwelling for Your Son, and Who, by Your Son’s death, foreseen by You, preserved her from all taint, grant, we beseech You, through her intercession, that we too may come to You unstained by sin." The Gospel is very on message, opting for the Annunciation recounted by Luke, during which Our Lady is called "full of grace," precisely what the Immaculate Conception means. The Angel's greeting is repeated in variation throughout the Mass to drive home the point.
The two downfalls I can see are that 1) the Mass began a process present in the reforms of Pius XII and Paul VI whereby every feast of Our Lady seems to use the Annunciation Gospel reading. This is particularly apparent in the new Assumption feast of the same Pius XII, which suggests Our Lady was assumed into heaven after her death not because her womb was holy and had born the Son of God (older Assumption Mass), but because She was "full of grace" (newer text), which is not wrong, but it is off message. 2) The other issue is that the new Immaculate Conception Mass came with an octave, which in turn supersedes the ferial days of Advent. This second issue was "fixed" when Pius XII axed practically all the octaves in the Roman rite, swinging this week from one extreme (during which the ferial Mass would not be said once) to another (no octave at all). Given that prior to St. Pius X all octaves were treated a bit differently perhaps there simply could have been a rule that the Sunday Mass had to be repeated on the first ferial or simple day, after which the octave could be observed.
The Immaculate Conception was a positive development in the post-Tridentine Roman liturgy, codifying the Church's belief, giving a feast some uniqueness, preserving ancient texts, and having some didactic qualities to its own.