I would say that, from the point of view of a trader, a digital currency has to satisfy 2 major conditions:
1. It has to be liquid - there has to be enough volume available, so the trader is able to get in and out of the position when he needs to without paying to much bid-offer spread. larger trading volumes also makes the currency more difficult to manipulate
2. It should offer volatility - it's easier to make profit trading the currency that moves up or down 2-3% within one day than the one that moves on average 0,2% per day. Having said that, trading the currency that moves 30% per day might be more like a gamble.
Other issues to consider are:
- are there tools available, such as charts, etc ?
- can you open short position or use leverage ?