In my experience, users can be divided into:
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basic content creators (can create and edit a page, news item, maybe an event)
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power users (can create forms, collections, portlets, content rules, etc., assign sharing permissions, group membership management, simple theme customization/tweaks)
What about system administrators (for the machine/OS and for the Plone install e.g. for patches and running buildout to add add-ons)?