I am using Alfresco everyday for this, and designing custom ECM solutions based on it is my job.
Scalable: Alfresco can easily scale to 1000+ users or much more. It is good at clustering.
Document Management Hosting: Yes, that's Alfresco's core feature.
Versioning: Of course. Can be enabled/disabled for a particular type/etc.
Workflows: Yes, one of the great strengths of Alfresco with the Activiti workflow engine included out-of-the-box.
Labeling: Alfresco has two ways to do this: tagging and categories.
Retention rules: For full-fledged Records Management there is a RM extension certified DoD 2015.02. Or you can just create rules.
Calendars: Each site has a calendar. I don't think roll-up view is possible though. You can also re-use .ics data and integrate with Google Calendar to a certain point.
Not sure what you mean by "publishing suite", but some of our customer use Alfresco for media publishing.
Wikis: Yes, in the sense of documentation that anyone can edit. It is not wikisyntax but a TinyMCE wysiwyg editor, though (which is better in most cases).
RSS or equivalent: Via this add-on. Activity feeds for your sites/teams.
Integration with MS Office Suite, Google Docs. And LibreOffice via CMIS.
Custom integrations are the real power of Alfresco. Web Scripts are easy to implement without restarting your server, for instance to implement your own REST APIs. For more heavy customization needs, Spring-based open source architecture allows you to modify anything with relative ease.
Phone/Email support with Alfresco Enterprise. From my experience, community support is the best among all ECM products.
Tasking is achieved via workflows, or issue trackers (an instance of Data Lists)
Integration for other tools: Yes. I would recommend the CMIS standard API so that your other tools stay product-independent.