They made it so that you could still use django's "render helpers" and things like the middleware directly, and just replace the 'render the template file' part.
Before you could always call a third-party rendering library, but it usually meant you wouldn't get things like the Django context renderers or template file lookup by default
Templating is the most frequently subbed out part of Django. I'm not sure how you'd make it more "pluggable."