A user-centric multi-site installation How do I go about creating a similar content-structure as wordpress.com? 


I want the user experience to only be of the sub-site (sub-site.example.com) and not of the main website (example.com). E.g. the home link goes to the sub-site, like this http://fiveandspice.wordpress.com/
In my case I will not offer blogs, but a web-application.
I'm thinking of utilizing Drupal for the user-system to handle the subscription-based accounts and generating the web pages.
The account-owners will have visits to their "page", and those visitors in turn would as well need account to do certain actions. Similar to how Basecamp work.
You can do this with the Aegir hosting System http://www.aegirproject.org/
It presents an intuitive front end to multisite.