Among the new Lync 2013 features that Lasko and other Customer Preview testers have discovered:
Full IPv6 support
VDI plugin, allowing full A/V support in virtual desktop environments
H.264 SVC codec support
Skype federation support
Persistent chat (or group chat) becomes a "full-fledged" Lync service, not just a bolted-on add-on
Audio and video added to Lync Web App
Consolidation of roles; no separate server role needed for monitoring and archiving
Hybrid deployments of on-premises and Lync Online cloud servers now supported (this capability is called "hybrid voice")
Support for M-N trunk routing, plus support for inter-trunk routing. Lasko explains: "This feature allows Lync to act as an intermediary between two or more different phone systems. For example, Lync can accept calls from one PBX, and pass the call through to another PBX. This can be very useful in larger environments and allows Lync to be the backbone of a corporate telephone network."
Support for a unified contact store, with information maintained in MIcrosoft Exchange Server 2013, enabling users to access/manage contacts from Lync 2013 Preview, Outlook or Outlook Web App
Lasko also noted a new feature that may or may not yet be in the Lync 2013 Customer Preview: A choice of ways to make audio/video calls using Lync from their mobile devices. Lasko explained: "Mobile clients will finally get the featureset people have been asking for. Now, mobile clients will be able to make audio and video calls from their mobile device using either a mobile data connection or wi-fi. Not sure if this will be available at launch or sometime after. I saw an early demo of Lync 2013 on a tablet running Windows 8, and it pretty much guaranteed I'll be buying a Windows 8 tablet when it comes out." (I tried searching through Microsoft's documentation for information on this feature, but didn't find a mention of it. Any testers come across this one?)
There's going to be a Metro-Style version of the Lync 2013 client, currently referred to as Lync MX. (There are two Office 2013 apps that will be available both in both Win32 Desktop and Windows 8/Windows RT Metro-Style flavors. The other is OneNote MX.) But that Metro-Style Lync client is not yet available to testers, at least public ones.