Some subtle nuances from Vito appear innocuous, but after seeing the movies time after time, I think they add to the richness of his character.
For example, when Brasi is stumbling through his prepared statement of gratitude at Connie's wedding, Vito's expression and slight body language suggest that he is sympathetic and ready to help his friend through this potentially awward moment.
He also subtly gestures with his hands to show his approval and gratitude when the baker proudly tells him about the cake.
While it took several times watching the movie to really pick up on this, I think it is a testament to fine acting and attention to detail to define a character.
Now when I see these scenes, I think this reflects a difference between Vito and Michael, who did not have his father's warmth.