The great thing about Unity (at least from my point of view) is, that all the scripts are compiled to machine code.
You can write relatively fast code with an easy to use and powerful scripting language.

I don't know how well the supported languages (JavaScript, C# and Boo) are implemented into the engine, though.

DX Studio for example has an excellent JavaScript implementation, where prototype based programming is fully supported.

In this regard ShiVa is still limited, as the scripting language is very high level.
As an example, it is not possible to define own objects (eg. data structures) in StoneScript.