Interesting observations, and now you've made me learn some things.
The RC regs on liturgical music have been deliberately confused by a bunch of wonk/termites (I'll not go into the rest of their personal, ah, problems) since VatII; but if one goes back to Pius X's writings on the topic, one gets a better sense of what prevailed from at least (circa) 1500-1965. B-16's writings are closely aligned to the work of Pius X.
Prior to VatII, there was a distinction between the 'High' (sung) Mass and the 'Low' (spoken) Mass--that no longer exists as it was known.
During the Low Mass, there was almost no singing, although there could be a hymn prior to (and after) the Mass. In the High Mass, the Proper Chants were sung, as were parts of the Ordinary of the Mass. The Propers were most often sung in Gregorian Chant OR in psalm-tone, and like the music Agrarian refers to, the Propers had a thematic unity with the rest of the readings of the Mass. In shorthand, it was 'tightly wound.'
However, in addition to the Ordinary and Propers, the choir could also sing voluntary motets which were not necessarily thematically-consistent with the Propers/Epistle/Gospel. Some of the motets utilized texts from Scripture; others used particular prayers (e.g., an Ave Maria or Panis Angelicus, respectively.)
Hymnody was restricted to popular devotions--not Masses. NO language other than Latin was permitted during the course of a Mass. The vulgar was allowed before or after a Mass ONLY.
The Bugnini/Weakland implementing commission made two significant changes: they erased the difference between the 'High' and 'Low' Mass, and allowed hymnody to be used during Mass, in the vulgar. IMHO, these changes must be examined carefully in light of B-16's work on Sacred Music (a glimpse of which can be inferred from the posted thread-head.)
Allowing hymnody has led to an even LESS 'tightly wound' schematic of worship than was present until 1965. The vast majority of hymns familiar to Catholics were not necessarily based on themes integral to various liturgies--rather, they were seasonal, Marian, or Eucharistic. These three classes, then, HAD to serve. Ironically, while allowing 'popular participation' the richness of each Sunday's theme-scheme was truncated, because current praxis does NOT require the priest (or anyone else) to sing or even recite the Propers--the Introit, Offertory, and Communion versicles. And because it is not required, it is not done, period.
So we have 'given a haircut' to the Mass; where the hair used to be specific and ornamental, it is now a military 'butch.' The best the iconoclasts can say is that 'there is still some hair there.'
Without extensive comment, the remaining hair has also taken on some characteristics which are unsettling--the equivalent of very bad purple or red-dye jobs...
I think that there is much to be said which is positive about the post-1500 well-composed Mass Ordinaries and voluntary motets. It is legitimate to argue that many of these demonstrate the Church's "universality," as a variety of cultural influences were 'baptized' into the service of the Church (for example, Durufle's 4 Motets, compared to Mozart's 'Ave Verum.') But as Ag. points out very well, the situation today is almost 'no bounds;' and it is confusing.
I'm away from my home computer, but will make some comments by way of describing how Orthodox chant works.
First, there is no high/low distinction in Orthodoxy. One either does the Liturgy or one doesn't. It is always sung/chanted (the terms are interchangable -- I prefer "chant", since it is a term that can't be used for inappropriate forms of music. One can hardly say that a choir "chants" a florid Italianate Russian setting of the Cherubic Hymn.
The position of the reader/psaltis/canonarch/chanter is still a very important one in Orthodoxy -- in the Greek practice in particular, there is a strong tradition of having one or two men who chant the responses, either leading the congregation, which joins in on the fixed and familiar parts, or doing it without congregational participation on the variable material.
The bottom line is that one doesn't need a trained choir to do sung services, although it certainly adds "polish" to the services. Timothy (Bp. Kallistos) Ware has a beautiful passage in "The Orthodox Church" that describes the reaction of an Englishman to attending a service in a little room in London where a priest, a deacon, and a solo chanter did a service by themselves that took his breath away.
My next comment is that older monophonic forms of chant, Western or Eastern, can easily be chanted well by one or two chanters, or by a trained choir. Once Catholic music made the turn into polyphony, it became the province of trained and even professional singers. There are parts of our services that should be sung by the trained singer(s) on kliros, and the Russians adopted some pretty florid stuff that requires a top-notch choir and isn't material on which it is easy for people to sing along. But in general, traditional chant forms in the Orthodox Church are actually fairly easily learned by the congregation over time with repetition. I would assume that the same is true of basic traditional Western monophonic chant.
As a side-note, when I browse through modern Catholic missals, the songs I encounter are just plain hard to sing. They use odd intervals, odd syncopations, etc... They look simple on the page, but I can only imagine what they sound like with a congregation trying to sing them. By contrast, we have chant melodies that look tough on the page, but in practice they are easily memorized and applied to text by people sight-reading material. Again, there has got to be traditional stuff in the ancient Catholic tradition that would do the same thing.
The other very important point that I touched on earlier is that the currently used Orthodox liturgical tradition that has done such a tremendous job of teaching and preserving the faith is not just our Divine Liturgy. Most parishes do Vespers on Saturday evening or Matins on Sunday morning, and many (like mine and Kolokotronis's) have both. There is a lot of fixed and variable material in these services. All has been "scrubbed" over centuries of use for beauty and doctrinal purity. The Orthodox parish norm is actually the monastic cycle. In practice, the entire cycle isn't done, and there are some abbreviations, particularly at Vespers and Matins, in many parishes. But the standard toward which we all look is the full monastic daily cycle.
Catholic parish practice has mostly devolved over the centuries to a single service. It is why so much damage was able to be done just by reworking that one service in the NO. Part of this is the result of having services that were not in the vernacular. But again, these services and hymnology very much exist in the Western tradition -- why aren't they being used? There are multiple traditional "canons" of the mass in the Western (and Eastern, for that matter) tradition. Why weren't those used if they wanted variety, instead of these newly composed things? There are countless pieces of patristic hymnology that are there, most of them translated into English. Why weren't they pressed into use. I guess I just don't understand...