You might not be able to tell by looking at it, but I use the same software, by DCScripts, that DU uses on this board, though DU has many, many tweaks they put into it themselves. You can vary the look of it to be fully expanded, like I have it, or compacted into strings in which you only see the subject matter of the OP as DU does it. You could have boards that are open to the public, to registered users only, to only certain invited posters, to only those designated in a certain category of users, to mods, admins, etc., including a combination of all of them. For instance, here at DU there is a Moderators' board that is only visible to mods.
Maintenance, at least for me, is easy. All I do is reconcile forum records and optimize the tables (two button clicks in the admin area) and that's it - and I suppose it would be the same unless your board grew so large that it was outstripping either your server or the bandwidth you are paying for with a server. Though my board is rather quiet now and just a place for old friends to gather, at one time I had to keep expanding bandwidth to the point where it just got too expensive and cut it back. So yes, it could "intrude the whole domain" if it becomes very busy and taxes the server.
http://www.politicalhardball.com/dc/dcboard.php