Some European countries tried out different systems in the past, for example the "Quick" system in Austria over a decade ago. As far as I know all of them failed.
Quick was a small chip on a smart card that allowed you to store a monetary amount and to pay with this chip at certain terminals. This worked essentially like real money: You had to regularly top-up your chip and if you lost it your money was gone.
This is all nice from theoretical aspects, but in practice it didn't provide any advantages to users. Why use your "Quick" card, when you can use your plain old debit card instead? With the debit card you earn interest, get a new card if you lose it, don't have to regularly top it up.
The same is true for this Canadian Mint thing: (As a regular user) why should I be interested? I can use my credit card to pay online and offline. I can use my online banking account to transfer money to friends. Where's the advantage?
One thing that you might not know is that in Canada, we don't have "debit cards" per se, but INTERAC cards, which can't be used for online purchases. This means that you must use a credit card for online purchases, which makes online transactions prohibitive for people who don't want or otherwise can't have credit card (i.e. kids).
I think the reason the Mint is launching this contest is to see what the community can come up with. Hopefully having a development community first will lead to enough useful applications that answer the question of advantages.
Here in NL we have "Chipknip" which is exactly the same as you describe.
The advantage this system has over debit is that the terminal does not need to have a network connection. So individuals can make small purchases at coffee machines, parking meters, etc. You do need to "top up" your card at an ATM-like station (Pin # required), but you don't have to enter your pin when you make a transaction.
If you loose your card, or your card breaks, you're screwed, which is why I don't like the system very much.