Microsoft Lync Server 2010 provides complete presence, instant messaging, conferencing and enterprise voice capabilities through a single, easy-to-use interface that is consistent across PC, browser, and mobile device. You can Architect your deployment for high availability using data center resiliency and survivable branch appliances. Administrators benefit from a single, consistent management infrastructure, new capabilities to increase availability, and interoperability with existing systems. Administrators also use a new PowerShell-based foundation for administration consistent with Exchange Server, Active Directory, and other Microsoft server products.

Here is a high level overview of the new features/improvements in Lync 2010:

• New User Interface
• The "Me" Area
• Contacts Enhancements
• Privacy Features
• Office and Windows 7 Integration
• Application Sharing, Desktop Sharing, and Collaboration Features
• Recording and Playback
• Improved Meeting Join Experience
• PSTN Dial-in and Dial-out Conferencing
• Video Enhancements
• Manager/Admin Enhancements
• Call Park and Retrieve
• Call Quality Checks and Diagnostics
• Improved Audio Device Handling
• Location and Emergency Services Support (E9-1-1)
• Call Handling: Delegation and Response Group Enhancements
• Enhanced Phone Experience

Here is a high level overview of the new features/improvements in Lync Server 2010: 

• Operating System and Processor Support
• Topology Changes
• New Management and Administration Tools
• New Presence Features
  • Enhanced privacy controls
  • Photographs in contact lists
  • Message waiting indicator for some phones running Microsoft Lync 2010 Phone Edition
• New Conferencing Features
  • Single meeting client
  • New downloadable meeting client
  • Meet simple URL
  • New Conferencing Modes
  • New meeting admission policy and controls
  • Enhanced meeting access levels
  • Assigned Conference IDs
  • Presentation Mode
  • Controls to Toggle Entry and Exit Announcements
  • Lobby provides improved user join experience
  • Access to DTMF commands during call
  • Recorded name for anonymous callers
  • Simplified leader join
• New Features for External User Access
  • Simplified installation
  • Streamlined management of Lync Server 2010 components
  • Consistent synchronization of internal and external Lync Server 2010 configuration changes
  • Improved security
• New Enterprise Voice Features
• New Monitoring Server Features
  • Richer Reporting including Dashboard View
  • Comprehensive Data for Usage Tracking and Voice Quality Diagnostics
  • New Management Features
• New Archiving Server Features
  • Archiving of IM content and meeting content consolidated
  • Searchable transcript of archived information
  • Per-user settings honored for conferences
  • New policy settings

You can download the RC from here:
http://technet.microsoft.com/en-us/evalcenter/ff808407.aspx