Built-in Integration with Social Networks
The hability to add social media buttons like Facebook's like or Twitt Me's on articles. It's mainly because there are several ones but none that allow full customization without gettin' into PHP code, and that's simply not Joomla!
Oscar Valbuena commented
It's great, for example, Joomla! may be like Hootsuite, integrated whit so many networks with feeds, and article post general.
AddThis has a nice extension that you can have displayed in a module position on every page. I created a module position that appears just above the main content where this displays, for sharing of the article (or blog on the current page) to any of 324 different social networking sites. It works well, and also tracks the sharing/recommendations.
I think this is really important, but I wonder if Facebook and the like are changing too fast for it to advisable to put this in core.
Agree, along the email, pdf and print button could be facebook, twiter, and other social network button integration. I think is better for SEO also (you know back-links).
All the best