First, they have to be backed up, which to say, they must have things of real value that all the currency derived from the inventory must be able to be exchanged for.  All currency should be able to be turned in, thus exchanged for the goods at a normal price and come out even.  There should be no left over currency or backed-up-inventory

Second, currency should be trusted.  Trust only comes from real investigations, in-depth research, audits and accountability.  Trust comes from real tests and openness.

Third, currency should have ease of transfer and acceptability. 

 Bit coins are missing the most important factor: they are not backed up by anything of real value.  Therefore it misses the second as well because there is nothing to audit.
They are working on the third and that will come with time and demonstration.