The runtime invocation is particularly nice for live debugging. You navigate in and out of program scope (using cd) and view variables in scope (using ls). It is all very smalltalk-ish.
Just have the different scopes and contexts attached to windows, and have XCode-like "inspector" windows where every text field is effectively another REPL, and you'd just about have Smalltalk. Ruby already has most of the cool language/library constructs, plus an order of magnitude more (too much) syntax.
