Grindlewald was unafraid of Voldemort...

I never paid attention to this small detail until re-watching the film tonight; we see Voldemort interrogate four people total in the film; the teacher in the beginning, Olivander, Grigorovich, and finally Grindlewald.

All of the previous three were terrified in a corner, pleading for their lives.

Grindlewald merely says he expected Voldemort and even gives him that wicked smile. And interestingly enough, Voldemort even asks him politely where the Elder Wand is, like he was his peer.

Even the most fearsome of villains in the franchise quiver at Voldemort's presence but not him.

This makes me look forward to his presence in the new Fantastic Beasts films.