I just want to point out that the "better integration with MythTV" is a big one. LinuxMCE unfortunately has a reputation for having a less stable MythTV than other distributions which include MythTV. The MythTV installed in LMCE is really no different than the one in other distributions, but unfortunately the "Network Control" feature which LMCE uses extensively to integrate MythTV into the rest of LMCE has had some problems in the past.
I created a patch which fixes most of the "Network Control" crashes, see here: http://svn.mythtv.org/trac/ticket/4115
There is still one more problem which Chewi alerted me to, where the "Network Control" handles digital tuners badly because it times out after two seconds on channel changes and gets stuck in an infinite loop trying to tune. The problem is that digital tuning can take longer than two seconds, especially in satellite setups where a rotor needs to physically point the dish at the proper satellite. I'll be looking at this problem as well.
For the HD DVD and Blue Ray question, basically mplayer can now play HD DVD and Blue Ray video from a non-encrypted disk. It does not have menus and you need a very beefy CPU but it does work if you do. If you are outside the USA, there are programs available which will decrypt the disk for mplayer use. Those of you in the USA will have to wait for this DRM mess to blow over.
So no menus anywhere, no encrypted disks in the US, and you need a very beefy CPU.
The VDR support is straight from the Pluto's sources, their developers are in Europe and wanted something that would work well for them. MythTV has a couple DVB developers in Europe, while VDR has practically all their developers there and they only do digital TV.