I know that there would be certain major differences, such as a Latin liturgy and the priest conducting mass with his back to the congregation, but are there any other, smaller differences? As an extra question, how did the earliest Christians practice, and when did something resembling a Roman Catholic mass first emerge?
Music historian here, so naturally I'm most interested in the evolution of the musical elements of the mass.
First of all, before the 9th century, there was no codified body of plainchant. Without notated chant, there was little musical consistency across regions of Europe. There were various regional rites (Gallican, Ambrosian, Roman, to name a few). During the 9th century, notated chant was disseminated throughout Europe at the request of Charlemagne. This body of chant (which has come to be known as "Gregorian chant") became the standard for the church. It was thought that unified settings of the mass would foster unity throughout the Catholic Church.
Another major difference between the mass in the 10th or 5th C and the 14th was the musical style. Before the 12th century, chant (musical settings of the mass text) were monophonic (one line of music). The Notre Dame School of composers (12th C) developed two and three voice organum. Over the 13th, 14th, and 15th centuries, the music becomes more complex. When the Reformation comes around, one of the complaints of the Reformers was that the music in the mass had become so ornate and florid that it was obscuring the meaning of the mass text.
I should also mention that certain musical elements of the mass Proper have gone in and out of favor over the centuries. Sequences, for instance, were very popular in the 12th C. During the 18th C, several prominent composers (Mozart, for example) wrote "Epistle Sonatas," independent instrumental works to be inserted in the mass between the Epistle and Gospel readings.
From the liturgical scholarship I've read, my understanding is that by the 14th century, the mass had largely crystallized into the form you'd recognize as the pre-Vatican II Latin mass.
The chief changes promulgated by the Council of Trent (hence the "Tridentine" mass) were 1) making the prayers at the foot of the altar (i.e. the confiteor "I confess to Almighty God...") required to be said at the steps in view of the people, 2) standardizing the calendar and propers (the parts of the mass that vary week to week and day to day) to have everyone use the Roman Missal, and 3) the removal of obstructions from the sanctuary that would block the view of the people.
Regarding 1) the confiteor and other opening prayers would often be said in the sacristy in private before the priest and other clergy came processing out, so the public mass would effectively begin with the Kyrie Eleison.
prior to Trent, every diocese had the authority to publish its own missal, so there were countless variations on which Saints' days were observed, the composition of various collects and antiphons, etc. Trent wanted to bring uniformity to address some of the complaints of Protestants about local saints of dubious nature, possibly heterodox usages in localities etc. Having everyone use the Roman Missal (i.e. The missal of the diocese of Rome) was the easiest way to bring about uniformity and orthodoxy. Exceptions were only made for those uses which could prove a continuous history of over 200 years. Thus, the Mozarabic Rite of Toledo and the Ambrosian Rite of Milan survive to this day. Note also, this was resisted by various dioceses in France, who stubbornly continued to publish their own "Gallican" missals into the early 19th century.
prior to Trent it was common for Western churches to have structures serving a similar (though not identical) liturgical function to the Eastern iconostasis. In England these were known as "Rood screens" and many have been reconstructed by antiquarian groups within the Church of England. In France they were known as "Jube'", in Germany "Lettner" and various other names. Trent wanted to make the various actions of the liturgy more visible and accessible to the people, so most of these "choir screens" were torn down as the Tridentine reforms spread. There are a few surviving examples, especially in churches that housed a cloistered religious community. Here's an example from Bruges, Belgium:
As far as earlier periods go, I've not read much. I do know, though, that the text of the Roman Canon (the Eucharistic prayer) was certainly standardized by the time of Pope Gregory I (r.590-604), to whom are attributed its last modifications prior to Vatican II.
The earliest recorded western liturgy I believe is that of Hippolytus of Rome which is usually posited as being from c.200-350.
You can read a translation of that text here, and it incidentally was an inspiration for the composition of Eucharistic Prayer II in the current Roman Missal:
http://www.liturgies.net/Liturgies/Historical/hippolytus.htm