A new feature of the latest ArcGIS Online update (December 2013) is that a simplified map viewer opens if the user has not logged in to their account. Capabilities such as Save, Add, Get Directions, Manage Bookmarks, and some layer options are not available in simplified mode, making it easier to use for casual, anonymous viewers of your maps.
Below is a comparison of what you see in normal (top) and simplified mode (bottom):
Note the appearance of Modify Map above search and adjacent to sign in. Click to return the map viewer to normal mode.
Signing in to your account is required to save the map if you’ve made changes. The Modify Map button does not appear if the map author has disabled saving a copy of the map. For more information see Save maps.