See, the funny thing about those markers is that they're so fluid and virtually impossible to agree upon due to regional differentiation. I for one don't think that 1453 was a watershed year either, for what it's worth. At least, not anymore. Further, what sort of "era" did the coincidence of the end of the Hundred Years' War and the capture of Constantinople bring? What similarity can be found between the end of the possession by an entity calling itself the Roman Empire of a single, somewhat important city and the end of a series of intermittent conflicts between the kings of England and the other claimants to the title of king of France? As to your other supposed "markers", the bubonic plague continued to break out in Europe after that date, feudally raised contingents continued to make up a large part of most European armies for quite some time after that date (and in Eastern Europe, it would last even longer, with the Russian service nobility obligation not even really getting started until this era and going strong until the 19th century, while in Sweden conscription and feudal obligations would be combined in the indelningsverk in the late 17th century...), and the existence of a class of persons referred to as the 'bourgeoisie' has nothing whatsoever to do with the date of 1453.Who said the 'datestamp' itself had to be a traumatic or wonderful event to the world ? When Constantinople fell it was little more than a glorified Venetian-Genose trading post. But it effectively marks the end of an era, just as the official end of the empty shell of a Roman Empire does, just as the accession of a semi-literate barbarian as first Holy Roman Emperor over a disorganized collection of fiefdoms marks the beginning of another. It's easy to minimize the event itself, but a judicious choice coincides with other trends. 1455 happens to coincide with the end of the Hundred Years War, when feudal cavalry no longer dominated the battlefield, after the bubonic plagues, and the beginning of the bourgeosie.
To extrapolate to the wider European situation: what changed in 800? Charlemagne was not crowned 'Holy Roman Emperor' in that year, and the institution that eventually became the Holy Roman Empire owed few of its institutions or reasons for creation to him and his government. His transitory empire did not start the reconquista, it did not oversee a sudden and fundamental shift in the function or performance of Western European militaria, it included territories that were heavily raided by Magyars and Vikings and did not herald either the beginning or end of such raids, and it certainly didn't coincide with the beginning of a "new era" in the Byzantine Empire, which didn't even finish its period of reforms and internal focus until the late 830s and early 840s with the military and associated reforms of Theophilos, which in turn weren't even much of a hallmark because the system he devised wouldn't be consistently applied with any success for another generation.
I dunno, like BL, I'm really, really leery of applying set dates to eras or pretending that they apply to anything more than highly restricted areas. Sure, the reign of Charlemagne was kind of important in some ways, but for most of Europe - let alone the world - it's basically useless, and its very transitory nature is a black mark as well. The nature of "dark ages" in various countries was different, so if you try to claim that they began to pull out of them it's silly to try to put them all into an Age of some kind or link them in any way - especially to an event such as Charlemagne being crowned "Roman Emperor" by a clergyman on a suspiciously convenient date - by necessity these sorts of categorization attempts become "some important or semiimportant stuff all happened in a few different places at around the same time with relatively unrelated causes but it was the DAWN OF A NEW ERA".