Mutual mistrust between buyer and seller needs to be supported. This is the toughest problem. Right now, sending Bitcoins is not tied to receiving something in return, and is irrevocable.
This is cryptographically solved in Bitcoin by use of a trusted third party. However, it is not widely implemented yet. (see:
https://en.bitcoin.it/wiki/Contracts)
The double-spending check system has to be as least as fast as normal credit card processing. Waiting minutes for the block chain to update is unacceptable.
This can be solved by having the coins come from a trusted source.
Some price stability is needed. Value shouldn't change more than 1% per week, worst case. 1% per month would be better.
Any new currency not tacked to an existing one will suffer this; I don't consider it an issue.
A better way of launching the currency needs to be developed.
What do you mean by this?