New features:

• Sign-in and messaging in multiple locations up to four at once.
• Signature sounds.
• Per contact sounds.
• SPAM Reporting.
• Animated Display Pictures - supports animated .gif files as display pictures.
• Link in Personal Status Message
• URLs in Personal Status Messages are hot and clickable in the Messenger contact list.

General improvements: 

• Behind the Scenes Changes 
• Architectural changes that modified how things work but shouldn’t have changed how things look. 
• Changes to sign-in, connectivity, presence, sending text IM’s, voice & video features.
• Windows Live Contacts Server
• wlcomm.exe runs in the background to keep your contacts’ information up-to-date and helps Messenger sign you in faster.