The retail penetration, however, is the barrier. If you can't use these things mostly everywhere, this service is going to be no more successful than Google Wallet.
The second issue is fragmentation. Look at wireless charging. Two separate solutions. What works at McDonald's won't work at Starbucks (and vice versa). Although both support wireless charging, they support different models. http://www.androidauthority.com/qi-a4wp-wireless-charging-st...
What use is a wireless payment service going to be if 90% of phones can't use it? When there is a competing standard?
Do Amex do a debit card? In Europe (certainly in the UK) debit cards are in much wider use than credit cards and Visa and Mastercard debit cards are given with almost every bank account.
It should be noted that they also offer regular credit cards, in addition to the charge card, and these do not require payment in full at the end of the bill cycle.
Edit: ignore me, I see somebody else has already mentioned this further down...
I'd still trade the charge card for their Costco version.
Almost all of my monthly spending is on my American Express.
However, you have to setup the American Express account separately, which probably has a lot to do with why many mom and pop type restaurants don't take it. But now that a lot of new restaurants seem to be using processors like Square, this is a complete non-issue.
I shop at farmers markets quite a bit, and a lot of the merchants there are using Square, and it is awesome =)