We did consider enabling popups on all layers by default. However, it would have been too much of a performance hit on the server. Also, our goal is for map authors to be thoughtful about what information they want to display to an end user. For a GIS analyst, they may be fine with a long list of attributes. However, a casual user would appreciate a more "authored" view of information.
As I mentioned before, we do plan on pushing the authoring of pop-ups into the item that references a map service, so when the item does get added to a map, it will already have a pop-up definition. Ultimately, we want to allow people to define it at the service level too. So when the service gets added to arcgis.com, it will already have a pop-up defined.
Thanks,
Mike