While I don't feel any one company has really nailed the location-aware deals space, when somebody finally does I think it'll be very closely tied to the payment. In my opinion, companies like Square and Dwolla have a distinct advantage that companies like Foursquare and Groupon (if they go this direction) lack.
I think the combination of payments, deals/coupons, and loyalty programs is what will really nail this space- but I agree with you, it's mostly the payments that will make or break this.
There are also some interesting opportunities with online advertising integration/tracking, but that's really just icing on the cake.
