Visual MSG is the term I use to describe elements added to films to give them enhanced texture, resonanance or mood. MSG, or monosodium glutamate is the food additive often described as a "flavor enhancer". Essentially it is used to activate taste receptors that trigger "savory" sensations. Commonly used by chinese restaurants.
Watching Sin City, it occured to me that the use of rain and snow was the visual equivalent of MSG. It makes the picture on the screen more alive and visually fuller just by giving the eye a layer of added visual activity to process. The stark, graphic black and white look is cool but may have been flat and monotonous without the rain and snow.
The other film that used a lot of visual MSG was Gladiator. Next time you watch it look at the use of fog, smoke, dust and most of all the use of flying dirt and debris in the fight scenes. A heavy handed use of "visual enhancers" that in the end works very well.