Mirroring of cards and lists is the best Pintask extension. It allows cards to live on multiple boards. Updates propagate to all mirrored cards at once. Useful for indicating dependencies between cards or maintaining an aggregate board for birds-eye view.
Nesting of cards and lists into other cards is the most powerful built-in feature. It allows you to have subtasks or indicate cross-task dependencies.
Hands-free time tracking is another useful Pintask extension. It tracks time for the first card on the "Doing" list, so when you move it to "Done", time tracking switches to the next card.