Shouldn’t unpredictably long seasons result in very different agriculture and society? Wouldn’t everyone try to accumulate as much grain as possible, because who knows when the winter’s gonna end? I imagine fluctuating shortages and surpluses. Only the biggest landowners floating, big guarded silos in every castle,extreme concentration of land and money. He who controls the Reach controls the Realm. Monetary surpluses leading to bubbles, commoners worked mercilessly to achieve that surplus.(tbc)

…Or, conversely, a more egalitarian,sharing system: to each according to his needs,or our much needed farmers will starve. But no,it’s all standard feudalism. Except: 1) Wintertown [but with no grain redistribution,I think] 2) Luwin advising lesser lords to set aside more of their harvest for the winter 3) Littlefinger acting like nobody has ever heard of supply and demand in all of Westeros. And nothing more.. So is my imagination wrong? Or didn’t GRRM think too much about the implications?

Yes, you’re quite right. In addition to the basic problem of how exactly you don’t suffer massive collapses in needed flora and fauna during the long winters, there’s a limit to how much you can store and for how long and for how long you can keep the livestock alive (which wasn’t that great in premodern society even with normal winters). So you’d be looking at not only starvation but diseases of malnutrition. 

There do seem to be fluctuating surpluses – certainly, the summers seem like a period of prodigal abundance and the winters as times of starvation. 

I think stores and silos are probably built standard in every castle and holdfast – indeed, a lot of fortifications were probably initially built to protect food storage. 

Regarding Littlefinger, I think the main difference is that the lords of Westeros wouldn’t normally countenance hoarding – yes, they expect prices to rise during the winter, but the idea that you’d artificially restrict supply in peacetime is out of the question. The nobility are inherently conservative, and one of the things they want to conserve is the labor force they need to work their land and the social order they need to keep that labor force under control. Littlefinger doesn’t care about the long-term consequences, but this is exactly the kind of things that set off peasant rebellions.