We talked about this a bit in the Gorby thread, but I think the biggest problem with communism is that it requires a proletarian revolution to be a stepping stone toward a communist utopia, but during that stepping stone phase, as the proletariat is consolidating and wielding state power, they are also becoming the new elite class. Once in that position, why should we expect them to be any more willing to yield their power than the bourgeoisie was? Communism fails because it can only succeed if humans in power make a conscious decision to act against human nature.
At least on a large scale. Small-scale communes that have worked probably do so because they are small enough that the sense of community overcomes the human desire to wield power over each other.