Advantages of Near Field Communication
Disadvantages of NFC
To the extent that quickly waving your phone encourages customers not to even bother looking at the screen.
There are some other potential downsides.
If the transaction receipt only shows up on the phone screen (paper will soon be a thing of the past, after all) and customers stop bothering to look, then there is the potential for routine errors to slip by uncaught. Customers used to tap-and-go might also miss out on coupons or other promotions that typically appear within an application on the phone screen. Obviously this isn't an important factor when you're riding a subway, but when you're buying a new TV it might be quite important.
The security implications of any RFID-based technology are also quite serious. Currently the RFID stickers used by some vendors to facilitate NFC payments are only as dependable as the adhesive on the back, and so if the sticker falls off your phone in a parking lot (which happens more than one might think), there's a risk that you could have just lost a lot of real money. Even if the sticker stays on your phone, a mildly-sophisticated criminal who knows what he's looking for could always read data off your RFID tag just by getting close enough.