When an upgrade occurs or a new plugin is added to a site, in addition to notifying the site administrator of database changes and parameters needing to be set, track the features per user and as each user logs in, notify the user of the new features, with an "accept" popup. Ideally, present this information to the user in the form of highlighted sections of the screen and popups describing the new feature. For non-core plugins, provide this tracking API as part of the install API.

See the latest version of Google Drive for an example of this done well.

Allow use of this feature for notification by the site administrator about new configurations, e.g. new reports created in a tool like Configurable Reports, or a new orientation course.