Solidarnosc was different primarily because it was a non-government organisation with a mass membership. Consider that, at the time, the Polish Workers' Party centrally organised all aspects of Polish society. It is true that trade unions are associated with the Left (broadly defined), but communist states bore little resemblance to the ideal "Left" of liberal democracies. In point of fact, in communist countries such as Poland there were not independent trade unions; if there were workers' advocacy groups, they were tied to the state apparatus and thus not truly independent.

So, then, consider this situation: the role of a trade union, if we boil it down to its most important components, is to represent the interests of the workers, and to prevent their exploitation at the hands of an authority (be that state or private enterprise.) While there were, in many cases, groups associated with industries that professed to be trade unions, they were sponsored and propped up by the state. Thus, they are compromised from the very beginning; how can they advocate for the workers' interests, when they have a vested interest in the state? The workers of Poland thus had no legitimate means of reforming their dire situation within the system; their advocates were not, in fact, their advocates (so to speak), and as Poland's national debt skyrocketed in the 1970s and early 1980s, working and living conditions became worse and worse. Pay rates lowered as costs increased.

In this context, when the general strikes took place around Poland in 1980, the reaction of the strike committee at the Lenin Shipyards in Gdansk was a huge turning point in Polish (and, indeed, Eastern European) history. Previously, strike actions had been dealt with individually; if certain groups of striking workers got their own way, they went off strike, and thus the state retained control of the situation. In this case, the Gdansk workers insisted they would remain on strike until workers around Poland achieved their objectives. Part of these objectives was the creation of Solidarnosc, a national, pan-industrial workers' union, without control or oversight from the PWP. When the Polish government backed down, and Solidarnosc was legalised, this was a watershed moment. For the first time ever, a mass trade union uncontrolled by the state now had responsibility and power. And because it was pan-industrial (i.e. not limited to one specific industry; any workers could join), the membership was massive. At its height, somewhere between 9 and 10 million Polish workers were members; that meant that about one-third of the entire Polish population had joined the only non-government organisation wielding any major political power in Poland. Under these circumstances, you can see why the already shaky foundations of the PWP were further undermined.

SOURCES:

Tony Judt, Postwar: A History of Europe since 1945

A. Brown, The Rise and Fall of Communism

C. Harman, Class Struggles in Eastern Europe, 1945-1983

Mark Mazower, Dark Continent

B. Misztal (ed.), Poland after Solidarity: Social Movements versus the State