>Anyone can change any program at any time, and see the changes immediately
Not sure what programming language Bret implementing the system but recently there's discussion how difficult and how slow to parse/compile some of the popular programming languages including C++ and Rust. In this case D is a unique anomaly where it has immediate rdmd REPL facility although it's a complex and a compiled language [1].
For creative, inventive, intuitive and comprehensive programming cyber physical system that involve hardware with fast sensing, control and immediate responses, D language is hands down the best programming there is [2]. The D authors however don't believe in any killer applications but this Dynamicland of communal computing most probably the niche that D is looking for to propel it for more wider adoption.
[1] Parsing Awk Is Tricky:
https://news.ycombinator.com/item?id=41422283
[2] D Features Overview: