Obviously the term »simplicity« is a term that can’t be defined in itself. Its meaning depends on what you apply it to. It is a term that qualifies a relation between activity, the skill and the required effort to perform or learn a task. It is not the object in itself, that is simple, but it is the usage that can have a “simple” quality.
The develmopment of the web in the last five years is a good example: there is an increase of activities users can do with no or only minimal effort or skill. Web applications are toolisations of data creating options for users to perform.
The Flickr Map (see my post yesterday) is a perfect example: The designers of that application have really thought through almost every possible interaction detail and provided clear interaction styles (e.g. How can I split a group of photos at a given spot? How can I add a group of photos to an existing location? How can one see that location on a given photo?). For the developers of the Flickr Map application this meant a lot of programming, testing and debugging. And this development cost was rewarded: 1,3 million photos were geo-coded in the first 24 hours.