For Eric Hobsbawm, it's crucial failure to apply technical innovation properly to the business of production.
Not unrelatedly, there was the Hayek-type idea that GOSPLAN was incapable by its very nature of having the knowledge required to allocate goods and services efficiently.
Various people pointed to the inability of the Soviet state to eliminate the allegiances of nationality and religion.
An interesting variation on this theme came from the late Ernest Gellner who argued not that the Soviet Union had eliminated the sacred but had, on the contrary, sacralised the mundane to an unbearable degree.
Then there was the question of pre-Soviet Russian history casting a shadow with it's tradition of absolutism and lack of any democratic experience to consider.
One could go on but thankfully we don't need to waste our time considering any of them because Neil Clark has the definitive answer:
"The problem with European communism was not that it was unsuccessful, but that it was too successful."The scales have fallen from my eyes.
(Hat tip: Harry's Place)