General:
* Countenance
* Visage
* Features
* Mien
* Look
* Demeanor
* Physiognomy
Specific:
* Grimace (pain, disgust)
* Wince (pain, discomfort)
* Smirk (arrogance, amusement)
* Scowl (anger, displeasure)
* Frown (sadness, worry)
* Grin (happiness, delight)
* Smile (happiness, pleasure)
* Glower (anger, hostility)
* Gaze (intense look)
* Stance (posture, attitude)
Figurative:
* Mirror of the soul
* Window to the soul
The best word choice depends on the context and the specific feeling you want to express.