The switch to KDE is intelligent, because if you take a look at plasma, it will bring some
lightweight nice looking UI Elements to be easy implemented.

At the moment it is a slow because it still has a big footprint of two systems running, I think .
I hope the two systems will do a good job and work together.

Plasma is great I tried it some days ago.

For Orbiters, a small KDE (Kiosk mode of KDE is great) should work really well in future.

The new KWin will also handle Hardware requirements better, so graphical effects will be disabled if the hardware is not strong enough.

I think XBMC has the greatest GUI around there.

But it is only a Orbiter, not a Server. So maybe the other way around might be interesting.
Use it as an Orbiter and integrate LinuxMCE.
I think it is the cheapest Orbiter you can get (without HighDef).

After the decission to integrate with KDE it would be really nice to integrate some of the "basic" kde apps, like Amarok and DigiKam as Library Backends.
Amarok and DigiKam are great in Editing, Sorting, Tagging the Files.

The cooles thing would be to have all Libraries in the local Network to be automatically configured and shared, so i always can access my media from anywhere in the house.

UPNP might be a good approach.