What is communism? Yoo's complaint has some merit: communists all too often preach to the choir and do not sufficiently explain communism and socialism to the audience not intimately familiar with the literature.
I myself am still a beginner, so take my interpretation with a grain of salt.
Communism is a political philosophy and, like all political philosophies, it evolves and is the subject of internal controversy and contention. Today's communism differs from Marx's communism, and the communism of someone in the CPUSA differs from the communism of someone in the RCP. In a similar sense, today's evolutionary biology differs from Darwin's, and Richard Dawkins' evolutionary biology differs from Stephen Jay Gould's.
Furthermore, there are two kinds of communism: idealistic communism, and "transitional socialism*". Idealistic communism is the goal; transitional socialism is the means to move towards that goal from our present capitalist/imperialist conditions.
*There is controversy within the community whether communism and socialism should be exact synonyms or whether socialism specifically denotes the transition between capitalism and communism. There's support for both views going back to Marx; my usage of socialism to denote specifically a transitional political and economic structure has enough support in the current literature that it is not at all idiosyncratic.
The goal of idealistic communism is uncomplicated: A society without any relations of exploitation: economic, political, social and psychological. A society without any relations of exploitation would not have classes, nor would it have a state in the modern understanding of the term: class distinctions reflect relations of exploitation, and the modern state exists to enforce and maintain class distinctions.
Marx understood — and every other communist in the world understands — that one cannot simply wave a magic wand and declare "no more exploitation": There must be a transition from our current capitalist relations of economic exploitation (and the political, social and psychological relations of exploitation that "sit on top" of economic exploitation).
When communists say, "True communism has never been tried," they mean that no society has achieved anything even close to an ideal communist society. When anti-communists say, "Communism has failed," they mean exactly the same thing. The difference is not in the evidence but in the interpretation of and conclusions drawn from the evidence. Briefly, anti-communists say communism fell; communists say that it was pushed.
What specifically do communists mean by "relations of exploitation"? Fundamentally they mean that under capitalism, labor power is a commodity, an item of exchange that obeys the law of supply and demand, where the price (exchange value) tends towards the cost. Since the use-value of labor power exceeds its cost, the surplus value of labor power tends to go to the owners of capital who employ labor power to produce physical commodities such as boots or DVD players. Labor power is thus exploited by owners of capital.
Labor power can be a commodity only when capital is not a commodity. An item of exchange can be exempted from the law of supply and demand only by the exercise of state power. It is trivially obvious from recent events that financial capital, the ownership of money, is presently exempted from the law of supply and demand by the US government's power to tax and create debt.
Likewise, labor power will not be a commodity only when (financial) capital is a commodity. It requires state power to exempt labor from the law of supply and demand. Since everyone owns his or her own labor power, an economic society where labor power is not a commodity is inherently more democratic than a society where only a privileged few can own capital. Therefore it should become increasingly unnecessary for state power to exempt labor power from commodity relations. The state, Marx conjectures, will "wither away" as an instrument for maintaining exploitative class distinctions.
The socialist transition from capitalism to idealistic communism therefore entails the state somehow exempt labor power from commodity relations. This goal requires that capital cannot be privately owned (unless it's "privately" owned by everyone). Historically, this exemption has been implemented by direct government ownership of capital. What's been missing historically is true democratic control of the state. Anti-communists typically consider this lack to be inherent to communism and socialism; modern communists typically consider this lack to be due to contingent historical factors that have nothing to do with the core ideology of communism. The communist interpretation is strongly supported by much of Mao Zedong's — uncontroversially part of the core canon of communist literature — post-revolutionary work, especially regarding the Cultural Revolution.
True democratic control of the state, though, is difficult to implement. The United States — indeed no Western so-called "democracy" — is not a true democracy: there are enormous social, psychological and legal/political obstacles to the people themselves actually making the day-to-day decisions that affect their own lives. These obstacles exist to privilege (in the literal sense of "private law") the owners of capital and exempt capital from commodity relations. The two-party system, the role of money in elections, the disconnection of the people from their "elected" representatives, all reinforce the de facto "dictatorship of the bourgeoisie." Indeed capitalism is specifically protected by the Fifth Amendment: economic power cannot be taken away by the people in the same sense that official political power can be taken away by the people.
The people have been indoctrinated for hundreds of years in the moral and ideological justification of capitalism, and indoctrinated for thousands — or tens of thousands — of years in the slave morality of religion. Simply handing the people direct political power would result in at best the election of a new ruling class; at worst it could result in chaos and sectarian warfare. (Anyone who thinks this fear is peculiar to communism really should understand the history of American democracy and the explicit concern of the founders to prevent "mob rule" and privilege the "best" (i.e. richest) citizens.)
It is not known that socialism really can transition to idealistic communism. It might be the case that humanity is doomed to some sort of relations of exploitation. But it is equally unknown that socialism cannot transition to communism. Furthermore, it is equally unknown that historical attempts to transition from capitalism to communism (in late 19th and early 20th century Europe) or from feudalism to communism (in the USSR and PRC or even Cuba and other "third-world" socialist countries) were doomed to failure: it is impossible to eliminate the implacable and absolute hostility of the Western capitalist countries towards the socialist countries as the primary cause of these failures. We will never know, for example, what might have happened in the USSR had they not been reasonably and justifiably afraid of conventional invasion by Germany and nuclear war by the United States.
Communists do not lightly advocate turning all our economic and political structures, socially evolved over thousands of years, upside down. It is, however, becoming increasingly clear that capitalism is failing, failing catastrophically, and failing precisely when it is unopposed and unchallenged by serious external or internal socialist pressures.