A few stats (from http://www.statisticbrain.com/wal-mart-company-statistics/): Wal-Mart, as a country, would be the 19th largest economy on earth. Of every $1 spent in the US, $0.06 goes to Wal-Mart. There are 100,000,000 shoppers in Wal-Mart every week.
NFC needs to become the way you expect to pay for things. Otherwise, it's Passbook -- that thing you vaguely remember that your phone can do, but that you don't really use, because in the time it takes to figure out if you can, you could just swipe your card.
If anyone's going to push it mainstream in one big push, it will be Apple pairing it with new iPhones. But it's not a given that that's the way it plays out, and every large retailer who pulls out is important.