Apparently you can't do a lot of things with it if you have tattoos on your wrist: http://www.reddit.com/r/apple/comments/344b3o/anyone_with_ta...
You're tattooing it wrong. But seriously, how could Apple not test this?
All jokes aside (I can live with the downvotes), I could see this easily falling through the cracks for a company new to wearables. What about hairy arms? What about skinny arms?