Poland was the first democratic multicultural state in history.
Rome? Greece? Babylon? Egypt?...
Most individuals under the yolk of Rome were not "citizens" and did not have the same rights.
Rome had democracy, so did Greece. It was limited by various factors (slavery, for instance), but Poland's democracy was limited too - you had to have blue blood to have any say. Millions of peasants working their landlords' fields had absolutely no say in any area of politics. That was one of the weakest point of early Polish democracy. The social status and general conditions of Polish peasantry created social divisions so strong that Russians (as an example) we able to use that in weakening Polish attempts of regaining independence. Their solution was simple - give Polish peasant just a little bit of what Polish nobles refused to give them through the centuries - a little bit of freedom, and inch or two of land. While this Russian Policy may have not been the deciding factor, it certainly helped - uprisings had limited participation.
All were welcome regardless of ethnic, religious, and racial origin.
That is true for the most part, but that also may have contributed to Poland's impotence at the end of the 18th century. For instance, it was better to be a Jews in Poland, than to be a Polish peasant. Jews had way more rights and privileges than millions of native Poles. Those Poles, while on occasion, forced to fight for one cause or another, weren't really the force to reckon with. They had no motivation. Jews were not subject to military service for centuries, and then, they did not consider Poland their father/mother land.
my $0.02 CDN (adjusted for inflation)