I find the inside the app to be easy for an intro but fundamentally limiting. I've always been excited by technology that features common, expandable data stores...
OpenDoc from Apple years ago featured some of this. BeOs's file system had some as well but it still gave you a hierarchical file system. WebOS has it some. The Newton had it. And I'm sure there are tons more examples that I never came across or don't come to mind now.
The problem w/ standard shared data storage for applications is that everyone has to agree on what a contact is or you need to be able to add additional data to it that other applications could tap into. Getting everyone to agree on formats, having them hold up over time etc is... problematic. Still, I hold out hope that pooled data storage will come into being.