1a)Without specifying which form of communism you are exactly discussing, you will never get a real answer to your question... that said, I will attempt to broadly address why I feel that Communism is in fact "bad."
1b) Value is subjective, not objective.. therefore, without a valuation system in place, like prices, true economic valuation is not possible.
2) If there is no incentive for profit within a system, there is no incentive to make that system more efficient.
3) You might argue that some communists want to pay people based on what they produce, but I would argue, as I mentioned, that would be impossible because there would be no real way to ascribe economic valuation.
4) Communism will never eliminate "ownership". Be it a committee or governing board, or a manager, someone is going to have control over resources that they can use to get favors etc.
5) Ultimately, the theory of Communism might sound fine, but putting it in practice is an attempt to deny human nature, and always runs into problems.