I generally agree with a minor quibble: There isn't an identity card that can be used as a passport. There are identity cards that are sufficient for limited travel and are unrecognized elsewhere. In the relatively widespread EU++ ID card system, at least in the country I live in, ID cards can't even be used in biometric immigration gates which require passports.
Americans can actually get "passport card" which functions like the EU national identity cards, within the United States. It is intended for use within the United States, as ID laws are changing. However, the RFID chip in the US passport card only has the identifying number encoded in it, for lookup in government databases. US biometric passports of course contain all of the traveler's information. Likewise EU national identity cards are biometric, as this is the standard.
With respect to the EU, as you know, there is to be a transition to biometric EU national ID cards, if countries have not already switched to them. The vast majority of countries already have. Some EU national ID cards are more useful than others, giving people online identities, for example.
Although I am culturally an American, I am also Croatian. I hold two citizenships. Croatia participates in the eID scheme [1]. Next time I go to Croatia, I am getting my eID, so I have an official identity on the internet. I am excited, as silly as it sounds.
