Well, usually site that has some sort of interactivity needs a:
Now, this needs to be extended:
Social networking (again, this depends what sort of network you are building) site needs to have:
- User profiles (and customization)
- Ability to add friends and to block people
- Sort of friend activity stream
- Media upload
- Sometimes a blogging or posting system
But, something like news portal needs to have:
- Easy way to add new content (articles, photo galleries, polls, etc.)
- Easy way for editors to edit the front-page and rearrange elements (like when there's some breaking story)
- Hierarchy system (basically this is the same permissions system)
- Photographer uploads photos to central gallery
- Journalist writes the article and picks some photos or gallery
- Editor approves the article and puts it somewhere on the frontpage
- Commenting system
- Categorizing (by categories and/or tags)
E-Commerce is similiar to news portal and social networking sites but it needs a payment system built in.
Also, some of the sites (like social networks) have APIs for external programmers that allows them to build other apps based around the informations from your site.
So, it depends on the project. You can have social network that has minimum functions like friends and feed, but again you can go complex as you want and you can add a lot of other features.
Also, some smaller news portals do not need a complex CMS system or advanced permissions, but it is neccessary when there are lot of people behind the portal.