Coin looks like a promising idea: store all your credit cards into one. 

But it is not free of shortcomings, mainly its dependency on a smartphone.  

As Christopher Mims pointed out for Quartz:

It sounds like a great idea… until you realize that the price of replacing a handful of plastic credit cards with a single one is that your ability to pay for anything becomes entirely dependent on the battery life of your smartphone. That’s for security reasons: If it loses contact with your phone for a user-specified period of time, from one to 10 minutes, Coin deactivates itself.

They will probably come up with some kind of solution for this problem at some point. Something else would be how Coin will ever going to be able to circumvent the regulatory maze of mobile payments outside of the US.