Probably the largest change in Ubercart 2.x is the replacement of the Workflow-ng module with a core module called Conditional Actions. When development on UC2 started, Workflow-ng's successor, the Rules module, did not yet exist. We needed a solution that would meet our immediate need for user configurable conditions and actions on events and be sustainable long term. As a result, Conditional Actions was born and integrated into Ubercart.
You can learn more about the system in the CA documentation.