When you are viewing a web map on ArcGIS online map, you can hit the "Share" button and it gives you the option for posting to Facebook and Twitter. We have had requets from internal staff to add an "E-Mail this link" type of button would be very helpful. The button could open the default mail client on the users PC, and maybe auto generate the Subject, something like "Check out this map" or whatever, then paste the link into the body, maybe even incvlude your own footer to give ESRI credit for the map generation
As an alterntive, if there was a note that said "Copy and paste this link into an e-mail to share this map" it would be helpful. We have to deal with the lowest common denominator when sharing information to the public, and while it makes sense to just copy and paste the link, it may not be that obvious to everyone.