Once more, Sakurai and his team's attention to detail in Smash Bros. never ceases to impress me.
When Link activates his Triforce Slash Final Smash, if you pause at the right moment when he's initiating the move and zoom the camera on him, you'll see him holding the Triforce of Courage in his sword hand.
This happens for a split second and the camera isn't very close to Link, but even so the development team included the Triforce of Courage in the special effects. You normally don't see this during gameplay.
This is a really cool detail.
Edit: This also happens with Toon Link. Another interesting detail is how the sword glows blue, possibly a reference to when Link deals the final blow to Ganon in Ocarina of Time: the Triforce of Courage appears in his hand, the sword starts glowing blue and he strikes Ganon multiple times before ending with a stab. The Final Smash also strikes the foe multiple times and ends with a stab...