What are the main differences between Unity and Unity Connection? or no differences outside the operating system.
Hope life is treating you well :)
The feature sets are getting closer and closer with Unity and Unity Connection. The doc below nicely encapsulates some of the differences. UC is not integrated with Exchange but can currently support up to 10,000 users (and this number is increasing - see below) Also, UC 7.0 supports Active-Active redundancy which was a stumbling block for many enterprise type deployments that require full redundancy. Another rather large difference is that Unity is a Windows based model and UC is a Linux based appliance like the 5.x, 6.x and 7.x Communication manager versions as you nicely noted.
For "full" Unified Messaging you will need Unity, for "Integrated" Messaging via IMAP access then UC 7.x is a great choice;
Cisco Unity-delivers unified messaging and intelligent voicemail capabilities to enterprise and mid-market customers with Microsoft Exchange and Lotus Domino environments
Cisco Unity Connection-combines integrated messaging, speech recognition, and call routing rules into an easy-to-manage system for midmarket customers with up to 10,000 users
The differences are nicley encapsulated in these docs;
Cisco Unified Messaging Feature Comparison
There is also an "older" (like me:) doc (attached) called "Selling Cisco Unified Communications Express". It gives a feature comparison between CUE, Unity and Unity Connection.
Hope this helps!