This thread is mostly to carry over some open musing in the history questions thread about the period of the Northern Wars (1558-1809), especially why the conflicts turned out why they did: first with Polish-Lithuanian marginalization and Danish-Norwegian decline, and then the slow eclipse of Sweden by the resurgent Muscovy/Russia and the new dynamo Brandenburg-Prussia. The relevant posts, and my personal responses to them:
Got a question: the conflicts around the Baltic in the sixteenth and seventeenth centuries had vastly different social and political effects on the countries that took part in them. Poland-Lithuania first formed a Commonwealth at Lublin, and then began decentralization, marked by the oh-so-devastating liberum veto. Muscovy had a somewhat dissimilar feudal system based on servitors, which was rather tumultuous (due to the nature of the autocracy), allowing first the depredations of the Oprichnina and then the Time of Troubles, but also some very notable military successes. Denmark-Norway had a slow decentralization during this time, begun by the strengthening of the Council during the Nordic Seven Years' War and continued to the end of the period. Sweden, however, used war as an excuse to grant the sovereign extraordinary powers in the 1540s and from then on established other centralizing institutions like that of the universal militia, the precursor to the levee en masse.

So the question is, why did each of these develop in its own way? This is part discussion, part question, and I have my own (half-formed) opinions but would definitely like to hear from others.
Why each country developed in its own way? There are many factors: in Poland it was relatively weak position of king bloodline because of continuous lack of male successors throughout ages. The whole process started with the rule of Casimir the Great(1333–1370), who left no successor. To claim a crown, his nephew Louis of Hungary made several agreements with noblemen that restricted the King's prerogatives in exchange for right of succession. The same situation happened after Louis death, and again later with the Sigismund II Augustus. In 1572, aristocratic families from Rus and Lithuania were extremely powerful. It was very easy for them to encourage the development of noblemen democracy, at the cost of country institutions and King's power.
Very poorly developed vast areas of Lithuania and Rus , and travel times (travel from side to side of Commonwealth took almost 1 year) made any resistance against legitimate ruler very easy.
I think a lot of this is extremely valid. But in some respects it doesn't help answer the question of why Poland-Lithuania was different. Sweden had plenty of dynastic problems, too; the episode of Sigismund III, erstwhile king of both Poland and Sweden, and his civil wars with Carl IX, is the most salient here. And Muscovy-Russia had the aforementioned Time of Troubles, the Smuta, which they still managed to recover from despite Polish-Lithuanian and Swedish pressure. But whereas these two states managed to escape from their difficulties with reasonably centralized governments, Poland had to grant liberties like the liberum veto to its nobles. When Sweden was pressed by Denmark and Norway in the 1540s war, Gustav Vasa was able to extract major concessions from the nobility in the name of national defense (which the nobility in the previous Swedish rebellions had been decidedly unwilling to grant; look what happened to Sten Sture). Why were the Polish kings unable to do the same during the Deluge? Was it the personal circumstance of having a truly terrible king in Jan II at the time? Or was it something deeper, something that the Polish nobles just didn't have that the Muscovite-Russian service-oriented nobility (what with their pomeshchiki and so forth) and the Swedish military nobility did?
The situation was very similar to Russian Time of Troubles, but Russian were in better position, because: they have not such strong nobility as in Poland and were less sensitive for foreign intervention, while Poland could be invaded from all directions.
Actually, I think that Poland's position during the Deluge assisted it in repelling the enemy. They were able to call on foreign support from the Habsburgs (who were terrified of a Swedish king on the throne at Krakow, only seven years after the Peace of Westphalia) and from the Empire, which proved crucial in helping to repel the Swedes back into their Royal Prussia quadrilateral. Sweden during the invasions of its own territory was unable to do any such thing, and was often faced by enemies from all sides. This might be a valid point for Muscovy-Russia's comparative survival during the Smuta, though; they were just too far away for the Poles to effectively reinforce their Moscow garrison without significantly more effort invested into the project, and Sigismund III cared more about getting Sweden back.
About Sweden-Finland, I would think the most salient feature of politics at the time was the triangulation of power between king-nobles-commoners. The usual link-up was for the commoners, who never actually lost politcal clout, to hand the king extraordinary powers to protect them from noble exploitation, something the king could then use to centralise power to himself.
That's fair, and a good analog to the other 'new monarchies' of the 16th century. One of the reasons it worked so well is that the Swedish military system, which relied in large part on alienations of royal land to the nobility to secure support (both manpower and generalship), was constantly "refreshed" by intermittent reduktions of noble land back to the royal house. It's easy to understand why the peasantry on royal lands was unhappy with the nobility, and readily allied with the kings in the reduktion debates: they had to bear the burden of both taxation and war, due to exemptions on noble-owned land. But what about the nobles? While it's true that some of them (like the infamous Magnus de la Gardie, the former favorite of Kristina) did oppose moves like the reduktion and the indelningsverk, by and large the lesser nobility followed along with some of the bigger magnates like Johan Gyllenstierna in championing the very reforms that would reduce their own holdings. My main question is: why were the Swedes so willing to do this, as compared with the Poles?

I also have somewhat related questions, like why did the Muscovite-Russian nobility subscribe to the ideal of service? More often than not the boyars and lesser pomeshchiki seemed to roll over to the autocracy's various outlandish and/or modernizing plots. Much is made of how Pyotr Velikiy did such a tremendous job transforming the Russian military from the shambles it was at Narva to the well oiled machine of Poltava (which is, as many such things are, an exaggeration; the number of trained, Westernized Russian regiments at Narva actually nearly equalled the numbers of the entire Swedish army, and the entire Russian force had seen combat more recently - against the Turks, for example - than the Swedish indelning conscripts). Anyway, Pyotr put through some pretty amazing reforms, and by and large the nobility bent to his will. Why were they more willing than the Polish nobility to abandon their liberties and traditional roles in the name of national defense?

And since that's kind of the same question as the first one, it'd be great if somebody cleared up why Denmark-Norway sucked so much in general. I mean, come on. So many epic fails over the years are just kinda inexcusable.

Sweden had plenty of dynastic problems, too; the episode of Sigismund III, erstwhile king of both Poland and Sweden, and his civil wars with Carl IX, is the most salient here. And Muscovy-Russia had the aforementioned Time of Troubles, the Smuta, which they still managed to recover from despite Polish-Lithuanian and Swedish pressure. But whereas these two states managed to escape from their difficulties with reasonably centralized governments, Poland had to grant liberties like the liberum veto to its nobles. When Sweden was pressed by Denmark and Norway in the 1540s war, Gustav Vasa was able to extract major concessions from the nobility in the name of national defense (which the nobility in the previous Swedish rebellions had been decidedly unwilling to grant; look what happened to Sten Sture). Why were the Polish kings unable to do the same during the Deluge?
Well, there are two major facts to be taken into consideration:
1) Sigismund III started aggressive wars in his own dynastic interest - thats why polish nobles weren't interested in giving up with their privileges and demanded additional reward for their engagement.
For the contrary, dynasty wars in Sweden and Time of Troubles for Russia were defensive wars which strenghtened their national identity and were an impulse for reforms.

2) Poland was broken by overextension: in the times of Casimir the Great Poland was quite well developed country with strong middle class, i mean townfolks and lesser nobility. In XVIth cnt, after joining in personal union with Grand Duchy of Lithuania, with its enormous amounts of unimproved land just waiting for taking by polish nobles, and in result of grain export boom, landowners gained privileged position. Unfortunately for Poland, they used it to cripple the political power of burgesses and peasants, what severely impaired economy, and in the long run lead to administrative and military decline.
There are also many differences between Russia and Poland: in Russia simple there was no tradition of noblemen freedom and rights at this time. Moreover, Russia's expansion was fueled by nationalistic ideas of uniting all Rus lands - at the beginning they had to regain them from Tartar's hand's, and later on as they pushed westwards, from Lithuanians and Polish. Surely, it was a huge motiavtion and kept boyars under tsars control, while in Poland nationalistic ideals were not so developed and strong until the fifties of XVIIth century.

In Poland, started by Sigismund Augustus and Stefan Bathory reforms, that could lead toward similar centralization as in Sweden or Russia, were abandonned by weak elected kings - as Henry of Valois, and aforementioned Sigismund III, who were more interested in his own dynastic affairs rather than condition of Polish state.

In times of Deluge, Poland had to fight in one time with two to four strong enemies (Sweden, Russia, Cossacks, and Ottomans) , part of them had came throughout modernisation already. Polish governing mechanism was corrupted by then, army exhausted, and economy wasted due to continuous wars. John Casimir was unable to make any serious reforms, whether he was totally incompetent or not - the situation was simply too difficult for him. Few good ideas appeared too late to have any chance for success - e.g. giving the Ukraine the equal status in Commonwealth as Poland and Lithuania, with all administration and provileges, or centralization, when all the nobles already blamed king for failures.

Conclusion: it was not single one bad decision, it was a few bad decisions taken in historical spin-points, that lead to Poland's decline.

That's fair, and a good analog to the other 'new monarchies' of the 16th century. One of the reasons it worked so well is that the Swedish military system, which relied in large part on alienations of royal land to the nobility to secure support (both manpower and generalship), was constantly "refreshed" by intermittent reduktions of noble land back to the royal house. It's easy to understand why the peasantry on royal lands was unhappy with the nobility, and readily allied with the kings in the reduktion debates: they had to bear the burden of both taxation and war, due to exemptions on noble-owned land. But what about the nobles? While it's true that some of them (like the infamous Magnus de la Gardie, the former favorite of Kristina) did oppose moves like the reduktion and the indelningsverk, by and large the lesser nobility followed along with some of the bigger magnates like Johan Gyllenstierna in championing the very reforms that would reduce their own holdings. My main question is: why were the Swedes so willing to do this, as compared with the Poles?
This would be a very short or very long answer. The real short one is that I think it worked both ways, i.e. that while the commoners sought royal protection from the nobles, the nobles otoh had reason to toe the line behind the king since they lacked the means to go it alone.

Oxenstierna's and Gustavus' reforms were pretty clear about turning the nobility into a social class of civil and military servants. I.e. the overhaul Sweden got by those to was very comprehensive, and it didn't leave that much of a role for a traditional landed aristocracy. It also helped that the nobles were internally divided, with the lower nobility quickly regarding the institutions of the state as their actual meal-ticket. I would also think it important that the Great Reduction could be made in a situation where the king, through a parliamentary decision, had been granted autcratic powers by the other three "états", i.e. everyone but the nobles.

Or put different; relatively weak nobles, relatively strong commoners, building a for the times incredibly strong state early.

There is one another aspect of XVIth century polish domestic policy: political conflict between federationists and supremationists.
The federationist wanted to create multinational and multicultural country with equal rights to three major nationalities: polish, lithuanian and rus. They were also supporters of strengthening state authorities and offices, and emancipation of peasantry and burgesses.
The supremationist treated Poland as dominant power in the Commonwealth, and denied the other, especially Rus, any political privileges. The were obscurant, nationalistic, and conservative, treating state as it was as a peak achievement in political development. Those "Sarmats", as they were called later, did a lot of harm to Poland in the name of bad sense of patriotism.
Unfortunately, the final output of this conflict was miserable: nether side was able to force their doctrine and conduct reforms, and Poland stayed in state of political isostasis.

What is especially interesting, some aspets of this argument returned in discussion between Pilsudski and Dmowski, in the first quarter of XXth century, and echoes even now in modern political debate in Poland.