Architecture features

Have been studying some architectural features and the following thought always comes up..
Disclaimer first; This post is based on my understanding.. as baby steps of understanding some arch and design concepts..

What makes the architecture
a) Extensible b) Robust

This post is about flushing out some practical details on above aspect..
so, lets start with "Extensible". The real question is how to design and validate the systems
are extensible..

Extensiblity means to extend already system for useful variant..
it may means
a) providing clear interface of components participating in system
- Eclipse provides plugin architecture to achieve this. There are extension points to extend
the behaviour.

- I believe COM did the same with home and remote interfaces.. correct me here!

b) The components playing in architecture should be loosley coupled
so what whould be loosly coupled

what next? ..

the post is progress..