- Centralized configuration of the individual wikis
- Centralized authentication and authorization
- Shared local files and resources
- Shared wiki files (i.e., images with ForeignAPIRepo)
With these goals in mind, there are numerous different methods of achieving all of these goals. Centralized configuration is simple enough: just have a CommonSettings.php or something similar, or use puppet or something.
Centralized authentication can be done either by using a shared DB so that the user logins are all the same, by using OpenID or some other authentication delegation service, or by attempting to use Wikimedia's custom SSO service E:CentralAuth. For most wikis, the first is the easiest option and the second is the best option, although it'd be nice if E:OpenID was a little more stable.