Adding support for QR codes requires some software (it's out there, and free usually) and a functioning camera with good enough resolution (again, almost omnipresent). NFC brings with it another chipset, another antenna and a chassis designed to have that antenna not mess with the others. If you want to do proper crypto with it, that's a rewiring of how you connect the SIM to your baseband to facilitate NFC-WI - I hope you're getting the picture here.
Geee is right about pairing being the other advantage asides from what I think are gimmicks like "bonking" to check into Foursquare. Realistically there is only a few ways to gain greater adoption: payments actually being deployed en-masse, or the incorporation of NFC chipsets into existing baseband modems.
Considering this is a service for London that number is pretty incredible.
But years ago TfL tried using NFC - a trial with the Nokia 6131. Ultimately it was rejected as it was too slow at the gates, a genuine concern in stations that see tens to hundreds of thousands move through it at peak times. Even now they still dismiss it as slow, as El Reg points out, despite the fact they don't appear to have actually redone any testing to confirm this.