Many people see QR codes, but many people don't know how to use them since an additional app is required. Sometimes TWO apps are required such as Zxing.
[2018 update: As of iOS 11, the iPhone's default camera app supports QR codes, and many new Android phones support QR codes through the built in Google Lens.]
Some newbie people open up a camera, and take a picture of the QR code, thinking it will do something.. and it doesn't.
Furthermore, some QR codes require specialized apps such as Bitcoin vs GoogleAuthenticator vs navigating to a website.
Lastly, if Google Goggles is installed, and a photo is taken, the user might get a notification of a "match" when it is uploaded to the cloud, deciphered, and sent back down.
Since some QR codes require apps, others have sensitive information (Bitcoin/Authenticator), and other QR codes are accessible through a variety of hacks, there are simply too many isolated uses of QR codes and I'm afraid that QR codes are too inconsistent to even consider.
What, if anything can be done to unify this fragmented, confusing, risky, error prone experience?