My read of the documentation also indicates a large overlap with linkerd. Hopefully one of the creators is around and can do a compare/contrast.
Linkerd is great technology but it is restricted to traffic management only. Istio provides a complete mesh that incorporates authentication and policy enforcement, in addition to traffic management and telemetry.
The Istio Auth subsystem provides certificate management and we are working on extending it to support authorization primitives as well.
The telemetry model is also different. Rather than having direct integrations with different metrics backends, we normalize metrics and pipe them through a single engine that can then re-route to any metrics backends (or multiple).
In contrast to Envoy, linkerd provides a minimalist configuration language, and explicitly does not support hot reloads, relying instead on dynamic provisioning and service abstractions.
Disclaimer: I work on Istio (on Mixer).
Our deployment model is a bit more transparent. Traffic gets transparently routed via Envoy, without using HTTP_PROXY or direct addressing of sidecars. This implies zero change to application code.
Secondly, Istio brings two more things to the table: policy enforcement (rate limits, ACLs, etc.), and authentication/authorization. Istio enables mutual TLS auth between services with automatic certificate management. The policy plane is extensible, where you can plug in adapters or specific policy implementations.