From Joe’s list, the most significant for audio are the new format support and the lip synch.
Personally, I’ve been waiting to upgrade from my Meridian 568 to a G68 till such a time as Meridian fully supports the 1.3 spec.
I’m also still using a CRT PJ (Sony G70) until digitals go to laser or LED illuminators (for full off, true blacks). Although my PJ supports HDMI 1.1 through an add-in HCDP capable card, which works very well with HD-DVD and my HD-TiVo, so I can wait a year or two more.
Also, need to wait for new BD or HD-DVD players to fully support 1.3 and the wider color spaces.
The biggest advantage HDMI 1.3 will confer is the simplification of our systems. Something for which I need to start aiming for, as I’m currently at the outer edges of the complexity envelope. A single cable from sources to processor, and from processor to amps and display devices is much better than the rats-nest many of us have now.