One element where I seem to have idiosyncratic assumptions that seem to be confusing my readers is my statement that capital must be concentrated to be efficient. I'm grateful to db0 for pointing me to Kevin A. Carson's Organization Theory: A Libertarian Perspective. I've just started the book (it looks like an almost 700-page slog; Carson is a wordy bastard who takes forever to get to the point), but just reading the first chapter has clarified my thinking and revealed some of my own unspoken, idiosyncratic assumptions.
It appears that it's an actual position on both the "left" and "right" that the maximal concentration of capital is good in and of itself, or is by itself a reliable indicator of the efficient use of capital. (Presumably there's a similar actual position regarding the maximal distribution of capital.) More concentration is in general always better than less concentration, and the only thing keeping us from One Big Corporation or One Big Government Bureaucracy, the most theoretically efficient use of capital, is our temporary and correctable administrative incompetence. This isn't my position, and my native cynicism should (but didn't) render me unsurprised that any rational person in the 21st century would hold such a moronic position.
It's completely my fault for not recognizing this trope in political economics, and completely my fault for not explaining my position in more detail. I'm pretty Zen and guilt-free about my faults, though, especially faults unavoidable as an autodidact; I do my best correct them as I find them, and move on.
When I say "capital must be concentrated", I do not mean that capital must or should be maximally concentrated, or concentrated as much as possible given other constraints, such as administrative competence. I mean, rather, that maximal distribution* is in all cases sub-optimal, and that some degree of concentration will always be better than no concentration at all. It seems, however, that even given perfect administrative competence, maximal concentration is also obviously sub-optimal.
*Where each individual literally owns and controls 1/6,000,000,000th of the world's physical and/or financial capital.
Furthermore, I definitely think it's a good idea on general principles, when and if it's physically possible, to distribute capital widely and concentrate it voluntarily. I would rather we go to the moon because a bunch of people want to, and they voluntarily pool their own capital to build a rocket, than because John F. Kennedy got into a pissing match with Nikita Khrushchev and appropriated the capital involuntarily through taxation in a capitalist state. On the other hand, I'm pleased we did in fact go to the moon, regardless of how we got there.
The present capitalist system, because it does indeed concentrate capital for its own sake (since an individual's personal ownership of finance capital is his relative standing in the capitalist class), cannot bring us to the point where capital can be distributed widely and concentrated voluntarily. All it can do is continue to involuntarily concentrate capital, until over-concentration finally brings down the United States, Western civilization, or all life on Earth.
My position as a communist is that we are not yet at the point where it's physically possible to perfectly distribute capital. We must presently concentrate capital to some degree to survive, and this concentration ipso facto cannot be voluntary; our only choice presently is how to involuntarily concentrate our capital.
As bad as capitalism is therefore, we cannot honestly say it is bad just because it concentrates capital; we cannot honestly say the remedy is to just distribute capital willy-nilly. We are forced today, rather, if all six billion people on the Earth are to survive and prosper, to find a different way to involuntarily concentrate our capital more efficiently, for our mutual benefit, and in such a way that we can move towards a wider distribution and ownership of capital.