I think that you have raised some good points.
However, I believe that a good number of these can be handled by the proper use of a netbooted machine.
Each user gets their own account, or just use the standard account that is linked to the media. Then each user could have his/her own music databases, video databases, etc.
About the only thing that you can't currently do with this scenario is netboot via wireless. I've been waiting for this to come down from Apple for years, as it should be absolutely essential for any kind of tablet device that they might ever release.
The wonderful thing about a netboot environment is that "reloading" the machine is usually as easy as restarting. Netboot is already included in OS X Server, and I'm really surprised that Apple has not leveraged it more in their enterprise offerings.
So a decent server could be an older G4, and a few clients could be Mac Minis with the local drives simply used as local storage, booting from the server based disk image. If they all use the same login, they can all simultaneously use the same media libraries. Sounds pretty good to me. I'd much prefer it over the Windows based system that I'm using now.
iMac Stepping Stone To An Entertainment Thin-Client?