I like to think that there are both data plane and control plane differences between most stacking implementations and Juniper's Virtual Chassis:
From a data plane perspective, each VC ingress port can directly address any VC destination port regardless of member location, and all features apply VC-wide (including features such as link aggregation). This is how traditional chassis operate, and is much different than simply stitching multiple boxes together in a ring.
From a control plane perspective, the traditional JUNOS master/backup routing engine architecture is preserved, and all members have a consistent view of the forwarding database. This provides a substantial improvement in management and availability.
Finally, the internal data flow uses a shortest-path, cost-aware and multicast-aware protocol, ensuring optimal use of the VC backplane resources and allowing multipath and extended reach topologies, allowing customers new degress of flexibility in network architecture, resulting in substantial capital and operational expense reduction...
Regards, Dave