The environment textures and lighting have seen significant improvements. The mansions and museums you are robbing now look more distinct and atmospheric. The shadow play is crucial in a stealth game, and the updated lighting engine makes hiding in the dark more immersive (and necessary) than ever.
Since the early v0.20 builds, the game has seen massive shifts, including a complete graphical overhaul that remains a point of debate among fans: Sneak Thief v0.20