The production and consumption of wine increased steadily from the pre-classical era to the rise of the Greek and Roman Civilizations and although Classicals from all walks of life drank wine in large quantities (by modern standards) there was almost certainly a vast difference between the quality and style of wine meant for the poor and what the wealthy and powerful consumed. Then Rome fell. From the (relative) chaos of centuries of migration, conquering and being conquered came the foundation of the modern wine world and the establishment of colonial boundaries that have a huge influence on the Old World vs. New World wine discussion we've entered into.
To be sure, medieval Europeans put their ancestry to shame in terms of shear consumption. Just like the Greeks and Romans everyone from serf/slave to the Kings and Religious Leaders drank fermented beverages (mostly wine, some ale) morning, noon and night. But w/out any major institution or government advocating any sort of temperance and the crumbled infrastructures unable to provide fresh water or sewage the vast majority of Europeans must have been in a continual haze. What didn't change was the quality to status/wealth ratio. The rich and powerful drank "good" meaning real wine that may have been spiced or otherwise manipulated but was probably strong and somewhat sweet. The poor drank mostly tart or bitter wine made from poor or under-ripe grapes or from the water fermented on the leftover pomace after the good juice had been pressed. There are always exceptions of course, and I'm sure some nobility drank complete garbage while the poor farmers and winemakers saved a bit of the best stuff for celebrations, but I think the distinction is largely accurate. Regardless, the presumption that the production of wine slowed dramatically during the early middle ages because of the lack of Roman expertise is probably wrong, although certainly trade was more difficult, technology and know-how stagnated or regressed and many prized vineyards fell into disrepair. So who comes to the rescue around 7 or 800 AD and begins to reclaim the best Roman and Pre-Roman vineyards and nurse them back to health and prosperity? The Popes and their legions of Monks of course!
Although the early Middle Ages saw decline in the extensive wine trade of Imperial Rome there was most likely an increase in local viticulture which included the specialization of indigenous varietals within their prospective regions in order to satiate the need and desire for wine. But most historians credit the various monastic orders with rejuvenating the best vineyards and preserving and progressing the practice of wine making. It does seem that much of what we think of as the "World's Great Wine Regions" owe much of their existence to the monks. A short list would include the Rhine, Mosel, Tuscany, Priorat, the Rhone, Champagne and of course Burgundy. A name conspicously absent is that of Bordeaux. Winemaking in Bordeaux was always a mercantile operation. This is because the region was acquired by the English Crown in the late 12th Century and during this time most of the wine produced in and around Bordeaux was sent to Bristol or London and is the source of Britain's love for claret. This wine came from modern day Graves and surrounding areas. It was not until the Dutch helped drain the swamps of Medoc in the 17th Century that the region was planted w/ vines.
By the end of the Renaissance the great monastic vineyards were in full swing, busy determining the best variatels to plant and the proper times to harvest as well as the most benificial ways of fermentation and storage. In the meantime Bordeaux was entering a troubled time during the 100 Years War and other political strife only to reemerge by the late 17th Century w/ the first wine to be bottled at the Chateau (Haut-Brion). Places like Jerez, The Douro and Madiera were doing fabulously well producing fortified wines for overseas journeys and English Nobility and adventurous monks were planting vineyards in North and South America. Communities in wine growing regions continued to make local wine for consumption and barter while more Northern towns and settlements increasingly took to beer (thanks to hops). Protestant upstarts began advocating temperance and literacy and common folks started thinking that maybe things could be a bit better.
The stage is set for the beginnings of modernity. Wine would continue to evolve as would wine drinkers and the forces of global trade, capitalism and nature would lead us to the the World Atlas of Wine and it's clear deliniation of Old World and New World wine producing regions.
(As state early I'll post a list of references used at the end of the full discussion).