Sorry for possible typos or sloppy English, feel free to ask any questions if I didn't make myself clear enough.

I would argue that the main reason of Deluge and predecessing events in Polish history being so notable is the character of warfare waged against Polish-Lithuanian Commonwealth. In late 16th - early 17th century PLC was arguably the strongest and most influential country of the Eastern Europe (maybe besides Ottoman Empire if we consider them to be eastern European power). It was a country with long and successful military and expansionist history, a country with both large coffers and relatively stable society. In comparison to PLC, Russia was in complete tatters after period of unrest and civil war which ended in 1618 and led to country being completely ravaged by said events, Prussia was direct Polish vassal and posed no significant threat, and Sweden, despite multiple attempts, wasn't successful in decisively defeating it. After winning Smolensk war against Russia in 1630s, PLC entered a period of stable, "fat", as some historians put it, existence, called Zloty Pokoj ("Golden Peace"). Said period would end in 1648, a year of large cossack uprising, which would in turn lead to PLC being besieged from all fronts by multiple opponents.

If we examine PLC's conflicts in previous era, then it's quite easy to see that majority of time said conflicts didn't lead to large damage inflicted upon Polish-Lithuanian territory proper. Sure, Lithuanian territory was ravaged by Russians in Livonian War, and Tatar raids were quite frequent and destructive to frontier lands, there were years of civil unrest and cossack insurrections, but nothing out of ordinary. It was a rare sight to see 17th century state not facing another rebellion every decade or so. In contrast to their neighbours, there were no big desolating events which could potentially pose a life-ending threat to PLC. There was no Russian army in Warsaw in 1612, quite the opposite. German lands lying to the west were in miserable conditions after the ending of Thirty Years War, and Ottomans also didn't achieve much in destroying Polish hegemony in the region.

Unlike previous years, both Khmelnitsky's uprising and Russo-Swedish wars of 1650s would shock PLC to its core. First of all, Polish army would face multitude of defeats against cossacks, essentially leaving adjoining lands defenseless. This led to said area being ravaged by rebels and allied Tatars, and was all the most shocking after more than ten years of countryside essentially having no fear of large scale warfare. Seeing PLC being unable to fully supress the unrest, Russian state would soon intervene in hopes of gaining sizable chunks of the hostile lands, same with Sweden.

In Russia, beginning of the new war was met with unmatched religious fervor and enthusiasm. It may probably be surprising by today's standarts, but differences in Christianity practiced by both countries (mixed Catholic-Orthodox of PLC and Orthodox for Russia) were one of the key factors leading to the conflict - that and also previous grievances and lost territories, which were taken by Poland in Russian Time of Troubles around 30 years earlier (roughly the same reasoning would later lead to Russian intervention in Northern War of 1655-1660 against Sweden). Military campaign against PLC, which began in 1654, was given the status of holy war, a crusade aimed at "liberating" Orthodox population of modern-day Belorussia/Belarus and Ukraine and "punishing" "Catholic heathens" for damage wrecked upon Russia in previous years.

Said military campaign came under unofficial unified motto of "bei shlyachtu, prinimai krestyan" ("smash noble men, recruit common men"): Orthodox population living under Polish rule in eastern provinces was encouraged by advancing Russian armies to join their ranks, flee from their masters, kill them or give them out to invading forces. Together with country as a whole being very tired from several long years of constant conflict, this led to peasants leaving their feudals or lynching them without any fear of prosecution. According to K. Bobiatynski, many Polish landowners expressed their concern, being fully aware of their subordinates' shaky loyalty. Polish nobility had to flee to the west in hopes of evading crawling threat, otherwise they would face very grim fate. This was doubled down with common laws of war (or lack thereof) with cities and fortresses refusing to surrender being given no quarter (like the town of Mstislavl which was almost completely destroyed).

Despite this, Russian armies generally tried to avoid needless bloodshed and excessive plunder since their primary target in early stages of conflict was bringing Lithuanian lands under Russian rule, not destroying them. This prohibition was largelly lifted after war started to drag on, and many provinces firstly conquered by Russians started to openly oppose them in hopes of rejoining PLC. Russian regiments, which conducted operations in western Lithuania in 1655, were given strict orders not to kill or enslave locals, with violators being punished by death. After towns in the area rebelled in 1659-1660, their attitude became much more cruel: rebels were considered to be oathbreakers and thus free from any invervention from the law. For example, successful siege of Brest in January of 1660 led to mass murder of inhabitants, both soldiers and civilians; their bodies were refused proper burial and dumped into the moat. Only about 50 commanding officers were spared and taken into captivity. Long, drawn-out military actions such as burning of Brest would lead to huge swathes of land left desolated. This would end in further economic difficulties, making PLC's situation worse with each passing year.

With wars waged against Poland by Sweden, Russia and their allies being very long and bloody, I personally think it was indeed very shocking to an average Polish citizen to suddenly come under very real threat of being killed, robbed or enslaved after period of time when PCL's land was mostly free from large conflicts.

Sources primarily used in editing of this reply:

1. I. Babulin, "Smolensk campaign and the battle of Schepeleviche 1654".

2. K. Bobiatynski, "Od Smolenska do Wilna".

3. O. Kurbatov, "Lithuanian campaign of prince Khovansky".

4. J. C. Pasek, "Pamietniki".

Let me give it a shot.

I'd say there have been three reasons for the Deluge to turn out so devastating for Poland; economical, political and military.

The first half of the 17th century was a period of pretty much constant warfare for Poland; while those wars never reached the, so to say, "core" regions of the Commonwealth (such as Lesser Poland, Greater Poland and proper Lithuania), they still put a dent of the country's resources. Out of those, the previous wars with Sweden have proven to be especially harmful; the Truce of Altmark and Treaty of Stuhmsdorf saw Poland lose their territories in Livonia, as well as (in Altmark's case) saw imposition of Swedish toll over goods flowing through Polish ports in southern Baltic.

Now, the main source of income for the Commonwealth's nobility has been agricultural production; crops grown in extensive estates across the Commonwealth had to be transported via rivers and exported to western Europe via sea. Losing a part of their income undermined Polish-Lithuanian economy. What made it even worse was the development of advanced forms of agriculture in western Europe - increased production meant less demand for imported grain, which further reduced nobility's income, and by extension, the resources at the Commonwealth's disposal. Combined with the fact that the noble "republic" was not very efficient when it comes to leveraging taxes or raising an army resulted in Poland-Lithuania already being relatively drained of resources by 1650.

Politically speaking, lots of ink has been spent to explain the shortcomings of the noble "republic"; I would try to point out to one element that has shown up during the Deluge specifically. With the death of Władysław IV in 1648, the ruling Vasa dynasty found itself in a tough spot. Jan Kazimierz, the last surviving son of Zygmunt III, was 39 at this point and still childless. The potential extinction of the Polish branch of the dynasty, as well as the fact that, as per rules of election, every nobleman could (in theory) become a future king, resulted in quite a few families considering their chances. One of them were Lithuanian Radziwiłł (Radvila) mangates, whose representatives, Bogusław (Boguslavas) and Janusz (Jonušas) openly expressed their ambitions. During the Deluge itself, they went as far as openly betraying the king, surrendering to the Swedish king and becoming his vassals.

Janusz's betrayal had more than just political dimension; as Janusz was a Calvinist and the invading Swedes Lutheran, this gave the Deluge a religious dimension as well. The fact that the Swedes were allied with Orthodox Russians, Lutheran Prussians and Calvinist Transilvanians, didn't help either. With the Polish Vasas being staunchly Catholic, the post-war mistrust between religions saw entrenchment of Catholicism and expulsion of several minorities, such as the Polish Brethren, who used to contribute towards Polish culture by being influential thinkers, writers and philosophers.

The very character of 17th century warfare contributed to this downfall as well; armies of that time relied on plunder and requisitions to supply themselves during campaign. The loss of crops and goods led to famine, which thinned population out. Apart from purely logistical aspect of that, the Swedish army also plundered palaces, libraries, churches and others, taking with them works of art, precious goods, documentation and so on.

So, to sum up:

• By the start of Deluge in 1655, the Commonwealth has already been a state exhausted by prior wars
• Its economy was both ailing from older treaties, and declining due to loss of demand for grain in western Europe (there was also little, if any, initiative to reform it, but that's a question for a different essay).
• The inefficient state apparatus had issues with raising an army capable of fending of the Swedish invasion.
• Frictions between the nobility resulted in whole armies surrendering and noblemen switching sides.
• The character of warfare resulted in grievous depopulation and devastation, further compounded by post-war expulsion of religious minorities.

I'm not sure there's too much to add in terms of military or confessional factors for this period being so devastating, but one element that's being overlooked is climate. The Deluge occurred during some of the worst years of the Little Ice Age, which manifested in colder and unstable weather conditions in Europe. The years 1641-6 in particularly were devastatingly cold, ruining harvests and causing widespread famine, combined with plagues of locusts in 1645-6. A decade later, two plague epidemics ripped through populations weakened by malnutrition, and the winter of 1657-8 was so cold that the Baltic froze entirely.

I think that aptly covers the 'calamitous and tragic' part of your question. But what does this have to do with warfare? As ever with environmental history, its difficult to draw a straight line between climate and human action, I'm well aware that 'determinism' is a dirty word. But Geoffrey Parker suggests that repeated failed harvests and the Cossacks' inability to feed themselves contributed to Khmelnytsky's decision to turn to war as the only way of supporting his people. We can see a similar calculation being made a decade earlier by the Manchu under Hong Taiji, whose invasion of China was in part prompted by adverse climate and the prospect of starvation. Obviously climate alone can't explain why Khmelnytsky waged war against the Commonwealth specifically, but its certainly a factor to consider at a critical juncture. Intuitively, warfare itself seriously exacerbates the effects of natural disasters; a community that can barely support itself in peacetime is going to be utterly devastated even if it is forced to support an occupying army, even if we put to one side the atrocities that said army might commit. What's so devastating about this period in Polish history, as elsewhere in Eurasia in the seventeenth century is the conjunction of natural disasters with human action (or inaction) that 'turned crisis into catastrophe' (Parker).

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Main source: Global Crisis: War, Climate Change and Catastrophe in the Seventeenth Century- Geoffrey Parker