Salt and water don't go bang! Why not?

A favorite chemistry demo is the tapping of a freshly cut slice of the metal sodium into a bowl of water to show it fiizzing about the surface as it reacts exothermically with the water.

Yet add sodium to water when it is combined with chlorine, in the form of sodium chloride, and it just quietly dissolves as the sodium and chlorine ions dissociate. Why is this? I assume it must be something to do with the rearrangement of the electons...