There are two ways socialism can go wrong. The first, as we have seen in the US, is the too-weak socialism of regulatory capitalism. While Roosevelt and Johnson did improve the general welfare of a lot of the people, and broadened incredibly access to surplus value relative to the narrow concentration of wealth in laissez faire capitalism, this too-weak socialism leaves capitalists too much power to propagandize and brainwash the masses of people and destroy what little protection they have won by political means, and even convince them to actively support not just laissez faire capitalism but even blatant kleptocracy. As Göring noted,
... it is the leaders of the country who determine the policy and it is always a simple matter to drag the people along, whether it is a democracy or a fascist dictatorship or a Parliament or a Communist dictatorship. ...voice or no voice, the people can always be brought to the bidding of the leaders. That is easy. All you have to do is to tell them they are being attacked, and denounce the pacifists for lack of patriotism and exposing the country to danger. It works the same way in any country.
The other insufficient kind of socialism is one that does not intentionally perpetuate class struggle. The most important lesson we have learned from both the Soviet Union and China is that socialism, while an important advance over communism, sets up a new ruling class. Socialism does not by itself end class struggle, it changes the class struggle.
It is possible to do worse than capitalism, and it is possible to do worse than capitalism while still maintaining the form and appearance of socialism. It is possible to reestablish feudalism under a nominally socialist society. Feudalism was, after all, in history the control of property by the government: it was explicitly the government of people who owned land, the critical property of that time.
I think this insistence on the right kind of socialism, as well as considerable disagreement on precisely what kind of socialism is truly the right kind, causes some friction between communists and socialists. "If socialism is better than capitalism," argue the socialists, "so shouldn't we get whatever socialism we can?" But socialism isn't necessarily better; some forms of socialism — not just governments such as National "Socialism" that just call themselves socialist — are worse than capitalism.
Even Marx has praise for capitalism. The establishment of "bourgeois right" under capitalism was an important innovation: it gave the bourgeois the power to struggle against feudal aristocracy by economic, political and ideological means rather than exclusively by violent, military means. And the need to first establish economic power independently of the political government (i.e. the proximate commanders of the army and the police) persisted even after the bourgeois won their class struggle, with at least some real separation between government coercion and economic activity. It's interesting to note that no capitalist country has ever created an explicit, direct plutocracy; even under the dictatorship of the bourgeois, the bourgeois still must act indirectly through propaganda and
This indirection, this institutionalization of political struggle and an external check on government power is one of the best innovations of capitalism and the bourgeois, and must, I think, be preserved and equally institutionalized in a socialist state even as the role of the private bourgeois as the primary opponent is eliminated.