Nice piece on user experience being more than just user interface.
The problem with delight is that we get used to it, we take it for granted, and pretty soon the inital wow effect wears off.
Keeping users engaged is a problem which has been solved, as Jenny Sundén points out in the piece, by game designers. I’d add a few television writers. And Apple solved it on their mobile devices by outsourcing it to hundreds of thousands of developers- each new app a potential delighter.