First of all, excuse my poor english.
Second, let me say that I don't know anything about CMSs.
My problem is as follows:
I must set up a site with a single domain, in which various editors/publishers/sellers can register themshelves, and after a subsequent approval by a "super-admin", they can begin to create their own content (forums, shopping, wikis,. .. etc), and distinguished from the other publishers (maybe different databases, or tables, or sub-folders for data)...
They will have probably different themes (or not), but, of course, they will share same site software.
Anonymous visitors should not need (if possible) login to visit the main site, nor any other publishers sub-sites.
They should have access to each of the sub-sites via menu and/or, if possible, via a general search by keyword (Lucene or equivalent) that return results from different sub-sites in a unique search operation. Clicking over theese results will give visitors acces to the different sub-sites...
That is, the content should be separated in different repositories, but I need search all of them in a single operation...
When I explained this I have said, "According of the characteristics of the different contents, the need for different user roles (authors and visitors), .. etc ... What you need is a CMS ....".