First off, Microsoft has really simplified the deployment of Lync over OCS. In previous versions you had to integrate settings and configuration inside of Active Directory (AD). This became a non-starter for most organizations, especially since you wanted the telecom people supporting users, not the networking people.
Lync offers a central management server that takes management out of AD and puts it in a place that is accessible to the right people. There is also a new Silverlight control panel that replaces the MMC for user management. This is a major step forward for adoption. There are also some really nice new monitoring features that are critical to something as important as enterprise voice. The new management pack for System Center Operations Manager is nice if you use that platform for monitoring, but even if you don't, the at-a-glance dashboard report built into the software will keep you on top of the health and usage of your systems.
I said Microsoft is aiming to kill the PBX and this is evident when you look at some of the new voice capabilities of the server. Previous versions of OCS were great for collaboration, but there were some seriously lacking features keeping it from being an enterprise solution. Lync Server addressed many of these in a big way:
Branch Office Survivability - Lync Server has the ability to monitor failures on WAN links at a branch office and re-route voice traffic through a local gateway to the public phone network. This resiliency is absolutely critical in an enterprise voice solution and Microsoft finally got it right in this release.
911-It may not be the first thing that jumps out at you, but Lync supports the enhanced 911 feature that the FCC has mandated for VoIP providers. It's one less obstacle to overcome when going with this solution.
Conferencing-Previous versions of OCS required another product (Microsoft Live Meeting) for conference calling and collaboration. Lync has it built in, and it's been improved over previous versions.
Common Area Phones - Again, it seems simple but they were not supported in previous versions of OCS. With Lync you can deploy them and very easily configure them to meet certain standards and restrictions.
Legacy Appliances - OCS lacked support for legacy analog phones and fax machines. Lync does not, so you can leverage the investment you have while making the move to a new VoIP platform.
Microsoft has been beating its chest about unified communications for years, but it lacked the one component that was needed to even get started-the phone system. With the release of Lync Server 2010, Microsoft has put itself squarely in the phone game. It might finally be time to put your old PBX out to pasture and integrate voice communications with the rest of your network.