Freedom of speech.
Anticommunist Sydney Hook believed that, unlike communism, socialism (or at least a welfare state) could only thrive with freedom of speech.
Still today? I suspect there are very few socialists who would recognize that distinction.
And as I said, they're all communists to me. Leftists, socialists, nanny staters, greens - whatever the specific cause du jour, any more I think they fall under a single umbrella.