The entire extensibility story in Unreal series has been amazing from the get go. Where QuakeC was really more of a scripting language "on the side", with UT it felt more like the entire game is written in UnrealScript, with some native bits here and there for performance. The language was interesting too, incorporating things such as first-class support for various states (you could define different implementations for a function depending on which state the object is, and derived classes could extend that).

No wonder the mutator scene in UT was crazy. My favorite mod for the original UT99 was Dr Strangelove, which modded the Redeemer gun (the one that shoots huge nuclear missiles) to allow you to ride them.