Below are some of the highlights of the 1.8.1 update:

• Ability to host games even when firewalled through a new metaserver proxy.
• Player pings will now be shown in the multiplayer game lobby (toggled by typing “.ping”).
• Support for Hi-DPI / Retina displays on Mac OS X and Windows.
• On Mac OS X, an improved full-screen menu experience that doesn't switch resolutions.

... And many, many more. A detailed list of changes is available here.