The bourgeois revolutions of the 18th an 19th centuries introduced a political paradigm as revolutionary as the economic paradigm of capitalism: the idea that political power (i.e. how we use police and soldiers) ineluctably and immutably belongs to the people. They can delegate that power, but it "cannot" be taken away or expropriated. This principle is directly stated in the preamble to the US Constitution: "We the people... do ordain and establish this Constitution."
Before the bourgeois democratic revolutions, political power was owned by the royalty and nobility. They "earned" it, and it was theirs to use as they pleased, for their own benefit. Any concessions they made to the people were concessions made to the substantial difficulty of keeping and exercising power in the real, objective world. To the extent the good of the people was any kind of goal, the good was supposed to emerge from the interplay of privately owned political power in the conflicts and struggles within the feudal hierarchy.
Essentially, bourgeois democracy socialized the private ownership of political power.
(Of course, nothing really changed except the ideas in people's heads and their distribution; people just changed how they thought about political power. But a human being is nothing but the ideas in his or her head, and our societies are nothing but the distribution of those ideas.)
One of the interesting features of bourgeois democracy is that the ownership of capital is specifically exempted from democratic control. A person may be deprived of his life or liberty on the due process of law, but according the Fifth Amendment to the US Constitution, he may be deprived of his private property for public use only on "just compensation". I'm not a Constitutional scholar, but I'm confident that the Supreme Court has consistently held that absentee ownership of capital does indeed constitute Fifth Amendment private property, and is exempt from socialization, even by due process of law.
The communist* project entails socializing the private ownership of capital, for precisely the same reasons that the bourgeois democrats socialized the ownership of political power. (More precisely, for the reasons the people threw their weight behind the capitalist class in their struggle with feudalism.) This principle and only this principle is the fundamental distinction between communism and capitalism.
*One important reason I call myself a communist and not a socialist is that too many people who call themselves socialist are not committed to the fundamental socialization of capital, preferring alternative fundamentals such as improved government regulation of privately owned capital. For all their differences and conflicts, most people who call themselves communists stand firm on the socialization of capital as a fundamental principle.
Everything else, including the concept of the "planned economy", are particular tactics and strategies to acquire capital from its private owners and to use it once it's been acquired. In much the same sense, a bicameral legislature and a distinct executive are particular tactics and strategies to implement democratic political power.
How should we actually socialize capital? How should we actually use and administer socialized capital? There are a lot of different ways to do so, from the extremes of anarcho-syndicalism on one hand, where individuals and small groups have more-or-less complete ownership of the capital they actually use; and on the other hand monolithic state communism where One Big Bureaucracy administers all the capital. I have a lot of specific ideas, but I do know: we're going to start off not just by making mistakes but by being half-assed; if we're smart, wise and lucky, we'll be able to correct our mistakes over time.
If we're not smart, wise and lucky, we'll fail, and someone else will have to try again later. In the late 19th and early 20th centuries, communism was just as new as democracy was in ancient Greece or the Roman republic; and communism is just as new today as democracy was at the founding of the American republic. The ancient Greeks and Romans — for a variety of controversial reasons — failed; the American republic did not. (Whether we succeeded, if one defines "success" as something other than avoiding catastrophic failure, is a matter of no small controversy.) Likewise the Soviet Union and China failed to socialize capital — again for a variety of controversial reasons — and descended back into capitalism.
The failures of ancient Greece and Rome did not prove that democracy was impossible, they proved only that particular strategies and tactics did not work under specific circumstances. When, seventeen centuries later, the structure and organization of feudalism — the private ownership of political power — became inconsistent and contradictory to material economic reality, the time was again ripe to make another try.
Similarly, the failures of the Soviet Union and China prove that specific strategies did not work under specific circumstances. Communism — the socialization of capital — is no more a panacea than democracy, and, like democracy, not all strategies consistent with the principle will be effective. We still have a real world to deal with, which imposes its own constraints independently of our political principles.
I have no intention of stopping here.
I do not want to say that socializing capital is essentially or by definition good, and that any and every society that socializes capital is therefore good -- or at least essentially better than any and every society that privatizes capital. I do not believe this principle any more than I believe that any and every society that socializes political power is essentially better than any and every society that privatizes political power. How we socialize capital is as important as that we do so.
I maintain, rather, that a society that efficiently and effectively socializes capital will, under present and foreseeable circumstances, be better than a society that efficiently and effectively privatizes capital. Furthermore, I maintain that we can independently determine this comparison: we do not have to embed socialization in our criteria to make this comparison.