Alfresco has feature parity with SharePoint on majority of collaboration and other types of features. If you have specific functionality you are interested in, I can provide more details.
As far as migration, any migration would be non-trivial for any system. You need to map content models together, understand how to map metadata, etc.
Relative to Web Content Management, we don't do just document management. Alfresco has a full-blown enterprise level Web Content Management solution. In my opinion, it is a more sophisticated platform than SharePoint, where MOSS 2007 is the first attempt at integration of Microsoft's Content Managment and SharePoint, and has many limitations for many cases.