It's interesting that the Soviet Union would invade Czechoslovakia in 1968, a nation whose leader Alexander Dubcek had grown up in the Soviet Union and who continued to swear allegiance to Moscow, while it allowed Romania to not only refuse to participate in the Warsaw Pact invasion and to condemn the Soviet action, but also to subsequently forge relations with the West on a scale far greater than what the Czechoslovaks had attempted. What made Romania special in this regard?

The crucial factor was that Romania simply wasn't geopolitically vital to the Soviet Union like East Germany, Czechoslovakia, and Poland. Romania bordered no Western nations (although it did border Yugoslavia), the Southern Carpathian mountain range made it a poor route for invasion of the Soviet Union (with Poland representing the historic corridor for Western invasions of Russia), and its people did not maintain the cultural connections with the West like the three nations mentioned here. It was an autocratic nation tucked away within the Eastern Bloc, it never promised to bring liberal democracy onto the Soviet doorstep (like Czechoslovakia in 1968 or Hungary in 1956), and it never threatened the supremacy if the Soviet military in Eastern Europe (again, like Hungary in 1956). The slightest anti-Soviet stirrings in a place like Poland threatened to undermine the balance of the Cold War, but Romania could officially recognize West Germany without making a splash in the history books. It's not that Romania was too weak, it just didn't affect Soviet security that much.