It looks like, in true Microsoft tradition, the third time is the charm. The new Lync Server 2010 unified communication server is not merely a relabeling of the previous Office Communications Server lineup -- it's real step forward in Microsoft's communication product line.
To me, the first version of Exchange 2000 that provided an in-house corporate instant messaging feature was pure genius. Microsoft pulled that feature in Exchange 2003 and started down a path that has led us to the Office Communications Server lineup of servers and apps. But it was very complex to deploy, making it a less than satisfying product. Now comes Lync Server 2010.
Lync Server: Easier to deploy, while being more granular
One of the first things I looked for in working with Lync Server deployment was the number of servers (physical or virtual) that I would need to get it up and running. Exchange, for example, has five server roles, but you need just one server (two is better). By comparison, Office Communications Server had an unwieldy number of required server roles and required too many servers to run them, so I was concerned about Lync Server's role count and server requirements.
The good news is that although Lync Server has even more roles than Office Communications Server, you don't need to install as many of them, and the net result is a reduction in server requirements. Note that alll of these roles must run on x64 hardware with a 64-bit edition of Windows Server 2008 or Windows Server 2008 R2, so you may need new server hardware and new server operating systems to deploy Lync Server 2010.
One nice aspect of having this higher number of finer-grained roles in Lync Server is that as with Exchange, you can deploy many of them together on one machine (aka collocation) or install them separately for larger environments that require each role to have more individual power. For the record, here are Lync Server's roles: archiving and monitoring, audio/videoconferencing, central management, director, edge server, group chat, Lync Web application, mediation, reach application sharing, survivable branch appliance, unified communications application server, and Web conferencing.