Different people have different ideas of serious. To me, exploratory programming is fairly serious, because that's the kind of programming that generates ideas.
Arc is already capable of supporting some subset of applications that are serious in your sense. News.YC is at least moderately serious in that sense.
You said News.YC uses some kind of persistent hash structure for storing everything. This seems to me like Greenspun's Tenth Law except with Berkeley DB instead of Common Lisp; I wouldn't want to write the logic to do what BDB already does much better and faster (I don't want to implement ACID transactions myself if I decide I need them).
