Out of curiousity I created this demo application to see how much data NFC enabled credit cards give up to a random reader. It turns out they give you pretty much everything you see on the front of the card.
Despite some news reports proclaiming NFC credit cards can be read by passing strangers, in my experience the card has to be in close contact with the back of the phone to get a good read.
Out of paranoia, this app censors credit card info and the content of "track 2 equivalent data" as I would hate to release something that could be used for evil.
This app has only been tested on UK cards, with a Galaxy Nexus so may not work on other credit cards. The app does not save or transmit any of the read data, but you use it at your own risk.