There’s a few links around the net listing some of the features coming in Exchange Online with 2010 but they are few and far betwee, so I thought it would be good to summarise them here.
The key components are:

• Voicemail with Unified Messaging
• Integrated archiving
• Retention policies and legal hold
• Transport rules
• Multi-mailbox search
• Conversation View
• MailTips
• Enhanced web-based administration
• Role-based access control
• Remote PowerShell
• Free/busy between cloud & on-premise
• Cross-premises management
• Native migration tools

Let’s have a look at these in a bit more detail.

Client Access

• IMAP
• Customise ActiveSync security policies
• Full OWA support in Firefox & Safari
• IM and presence in OWA
• OWA side-by-side calendar view
• Mobile free/busy lookup
• Mobile reply status
• Mobile SMS sync
• Over-the-air update for Outlook mobile

Email & Calendaring

• Conversation view
• Ingore/move conversation
• MailTips
• Mailk aggregation
• Shared nickname cache across OWA & mobile
• Message tracking (user self-service)
• Free/busy interoperability with on-premise
• Free/busy & calendar sharing with other organisations (ie. Exchange federation)
• Publish calendar for anonymous access

Contacts & Directory

• Dynamic distribution groups
• Restricted distribution groups
• Moderated distribution groups
• Share personal contacts
• Contact information update (user self-service)

Unified Messaging & Fax

• Hosted voicemail (integrationon with PABX)
• Outlook Voice Access
• ‘Play on phone’ button
• Missed call notifications
• Caller ID
• Company auto-attendant
• Self-service PIN reset
• Message waiting indicator
• Voicemail preview
• Call answering rules
• Protected voicemail

Security

• Sign in using AD credentials
• Route outbound mail via on-premise for filtering
• Multi-factor authentication (smartcard, soft certificate)
• Integration with on-premise RMS (for search, pre-licensing)
• RMS transport rules
• Journal decryption of RMS messages
• Outlook Protection Rules

Administration

• Remote PowerShell
• Role Based Access Controls
• Exchange Management Console
• Enhanced migration tools
• No OST re-synchronisation after migration
• Offboarding tools
• Support for organisations greater than 30,000 users

Compliance & Archiving

• Exchange Personal Archive
• E-Discovery (Multi-mailbox search)
• Message Retention (Archive & Delete policies)
• Message Retention (Legal Hold poliy)
• Compliance auditing
• Transport rules
• Custom disclaimers