I'm not particularly in love with his paintings (history paintings, mostly....), but I do find his studies on how different animal traits could be expressed in the heads/faces of humans rather intriguing. It's like Animorphs of the seventeenth century (I never actually read any of those books, but I remember seeing them at the library and being fascinated and vaguely repulsed by the pictures on the covers)!
The following physiognomic heads inspired by animals, 1670:
Le Brun also had some very interesting ideas about facial expression and eyebrows. Descartes, with his ideas on dualism, believed that there is a nodal point between extension and mind - the pineal gland. Le Brun expanded on this point:
"And as we have said that the gland which is in the middle of the brain is the place where the soul receives the images of the passions, so the eyebrow is the part of the face where the passions are best distinguished, alttough many have thought that it was the eyes."
good bit on Le Brun and French painting - "The Legible Body: Le Brun" in Word and Image: French Painting of the Ancien Regime, by Norman Bryson (Cambridge University Press, 1981).