Not only could peasants reduce their obligations to their estate holders by paying taxes directly to the Swedish state, but the Swedes developed what passed for an admirable system of education, so you had a level of literacy developing that tended to help nationalism along.
Is nationalism what we want here?
I don't know enough about the nature of social structures and linguistic issues in Poland-Lithuania to speculate about what could have happened there, but it seems like in OTL the eastern areas tended to end up with a Polonized upper class and a backward peasant class speaking the local language(s). Not much of a running start for what would have had to be a big flat Switzerland to be viable politically.
Okay, this I can talk about, and will in a bit.