I could probably write a book on what I love about Simon's acting style, so its difficult to know where to start.
Eyes - yes definitely, very expressive eyes. He's able to portray incredible depth of emotion, so that it appears to come from feelings deep inside him. A comment I saw that I completely agree with is "when Simon looks sad, he looks sad through to his soul". By contrast, he can also do lovely sparkly, twinkly eyes that help to make his whole face light up.
Subtlety of expression - yes I agree, he can communicate without a script. He seems to have complete control of his facial muscles, so that the tiniest movement can tell us what he's thinking or feeling. I think The Guardian demonstrated this talent perfectly, Nick didn't always say a lot but Simon used the silences to great effect.
I also love the way he uses his hands - beautiful sensuous hands.
And his body language is also expressive and honest. I think Simon is a very observant, empathetic person and he uses this in his performances. A wonderful example of body language, is his first kiss with Lulu - I think the whole scene is mesmerising. And when they hear Brian, the kiss is broken off but Nick's body is still leaning towards Lulu as though he can't bear to tear himself away.
I have to give a brief mention to his voice - I love the crack in it that we sometimes hear.