what is the downside of communism ?

The downside is the naive belief that the average human is to be excepted to use humanitarianism and goodwill as a means of motivation to do work and provide for the economy. Anyone even remotely experienced with human nature knows the chances of that happening are slim.

The downside is that Communism doesn't work for large nations. Villages and small tribes, yes. Large countries like Russia and the like, no.

Marx failed to realise that people aren't ants who will sacrifice all for queen and anthill. People are naturally competitive and greedy. Add that there are no personal freedoms under Communism and you have a royal mess. The Soviet Empire lasted about 70 years; that ought to prove to people it doesn't work.

Which song are you referring to? These?

http://www.youtube.com/watch?v=0yDrtNEr_5M

Soviet National Anthem(With Lyrics)

http://www.youtube.com/watch?v=suVB3YGIUk0

The Communist Internationale (Original, with English Lyrics)