Communism is not democratic... not even in theory. It involves the communal ownership of all assets and the distribution of such assets duty of the state. However, the state is, as others have put it, not very good at doing such things with any degree of success.
Here's a good economic term for you, Purchasing Power Parity (PPP). Basically, PPP is an economic measure of what one currency will buy in another currency with respect to a basket of goods. Another good term to know, if you are going to argue such things, is GDP/capita in PPP (GDPP). This is the total amount earned in or by an economy divided by the number of people who participate in the economy, adjusted back into the currency in question via said basket of goods.
With PPP, we say, "I have $100 US, what can I buy with them in China, Belgium, Germany...?" Then we say, "I have 100 Euros, what can I buy with them in China, the US...?" And we keep doing this for all countries. With GDPP, we say, "OK. I am in the US and earn $X US each year. In terms of US $$, what would my yearly earnings be if I were a citizen of China (or wherever) and went to the US (or wherever) to purchase goods?" Now that we have our definitions out of the way, we can proceed with a very unhappy lesson.
According to the CIA World Fact Book:
The US GDPP was $47,500 (2008 est.) and inflation was 3.8% (2008 est.). This means that on average, US residents contributed 47.5K in 2008 to the GDP and will require a 3.8% cost of living raise in order to keep from losing ground next year in terms of what goods they can purchase with a constant sized basket for a devaluing (inflating) dollar.
Let's look at another Democratic country, Belgium. Their GDPP was $37,500 (2008 est.). I'm in the US, so I use $US... use what you wish. If we took the average contribution of one citizen of Belgium, and compared it to what their contribution would buy if they came to the US with it, they would be in pretty good shape. $37K is nothing to sneeze at.
Now for the tragedy:
Russia - whether or not they were "Communists" by one definition or another or were not is immaterial. They were attempting to implement a planned economy, and in economic terms, that is communism. The GDPP is $16,100 (2008 est.) which sounds great, except that there is 14.1% (2008 est.) inflation. This means that every year, your 100 Rubles buys you 14.1% less. This is a symptom of an economy that is hurting, badly. Put another way, more bluntly, is that the average amount added to the economy of Russia would be nearly at the poverty line in the US. Russia has great (staggering, really) reserves of resources, it will recover with time and with market freedom... provided that the Chinese do not invade them first.
China - Currently practicing Communism with "Autonomous Economic Areas" (or something like that). The Chinese GDPP was $6,000 (2008 est.) with inflation of 5.9% (2008 est.). Acceptable inflation numbers for an emerging economy, but guys, that is abject poverty.
Laos - GDPP was $2,100 (2008 est.) with 8.6% (2008 est.). Tragic? Yes.
Vietnam - GDPP was $2,800 (2008 est.) with 23.1% (2008 est.) inflation. Pitiful? Again, yes.
Cuba - GDPP was $9,500 (2008 est.) with 3.4% (2008 est.) inflation. Don't get too excited yet. It goes up each year that Castro is out of power and given that Venezuela keeps providing them with subsidies in terms of fuel and the Chinese keep providing them with subsidies of everything else (with free stuff, you can buy a lot).
The thing is that communes (of any type) seldom work and people keep trying communism despite the fact that it fails and does so in a way that is heart wrenching. People have used communism because it sounds very seductive to some people, as a means to grab power and to hold that power... nothing more.
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Edit 1: Grammar
Edit 2: "capital" != "capita"
7 x 13 = 28
Economics today is a race between economists striving to build bigger and better idiot-proof economic or financial models or programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning...