So Finland was never knocked out of WWII. It ended the war with an undefeated army. No doubt the Soviets could have taken it over, but the Finns had made certain the Soviets thought it would cost more than it was worth, which is precisely what any small state with Great Power neighbours can hope for.
Stalin could be quite pragmatic and even seemingly magnanimous, when he felt it served his interests. He also not only withheld help for the Greek communists in the 1946-49 civil war, for instance, but also forced Dimitrov, Tito and Hoxha to withhold aid as well. There are today in central Hungary, living around the town of Dunaújváros (old "Sztálinváros" in the good old days, ironically), a large cluster of Greek communist refugees (and by now, their descendants) from the civil war who are still quite bitter about Stalin's lack of support.
But in any event, Stalin "spared" Finland the fate of becoming an "S.S.R." or "Народная Республика" simply because it was not in his way and he did not want to waste forces pacifying something with such little value for him. Finland's threat in 1939-40 was not Finland itself but the fact its borders were so close -- within artillery range -- to Leningrad, and that in any Russo-German conflict Finland was likely to take the German side, so that the Finnish borders served as a potential host to a Wehrmacht threat. In 1945, with Soviet troops in Berlin, Finland posed no such threat and therefore could be allowed to go. As Stalin correctly surmised, the long Soviet border with Finland in Karelia served as a sufficient reminder to Helsinki that it best not act too hostilely to Soviet interests, much as Hitler, with Norway occupied, did not have to actually occupy Sweden to get it to cooperate with his interests. Though a free country in most respects, the Finns throughout the Cold War were infamous among their Scandinavian neighbors for muting any criticism of the USSR. During the Chernobyl disaster, for instance, Swedish scientists argued for crucial weeks with their Finnish colleagues about alarming radiation readings, with the Finns only cooperating after Gorbachov publically admitted that there was a problem at the Chernobyl nuclear plant.
In Poland, it was known and accepted -- to a certain extent -- that the Soviet occupation was not going to go away and Moscow would never trust an independent Poland, but Hungarians (who do not have a long, hostile history with the Russians as we do) dreamed after Stalin's death in the 1950s of a "Finnish solution" for Hungary, whereby Hungary would be neutral and promise to behave in exchange for Moscow withdrawing its armed forces and allowing Hungary more independence. After the 1955 Austria Treaty, the focus switched from Finland to Austria, and Hungarians hoped their treaty day would come... but alas....