I like the condition system. I knew it from Lisp, but I never thought about it consciously. For interactive systems, it seems superior to stopping execution due to an unhandled exception.

It's similar to the SmallTalk debugger asking you when it encounters an unknown symbol, allowing you to for instance correct a misspelling.

These ideas are so logical and long known, I wonder why they aren't used more. (Are they used somewhere outside the respective languages at all?)