Since you have worked with Sharepoint before, you understand many of the basics of document management, etc. Sharepoint is more of a finished product, while Alfresco is more of a kit with some basic capabilities. You will need to do quite a bit of reading, followed by doing, in order to understand what Alfresco might be capable of. I strongly recommend reading through one or more of the books on Alfresco (3.0 or later), as the authors will be able to tell you things you need to hear.
My suggestions:
Take it slow. Content management systems can crash and burn if the technology and the audience aren't ready for each other.
Get pilot projects going, the simpler the better, and continually solicit feedback from those projects.
Establish a governance structure in the organization. Explain that the technology can only help to enforce business rules, but it cannot write them nor audit them.