The 'outdated' bit has more to do with the themes than limitations of MythUI. MythUI does support some animation, it's just not all enabled or used in themes to any significant degree currently. The library was designed from the ground up with animation in mind though. As the developer who put the most work into MythUI over 18 months of development I can be a bit sensitive about criticism. What we really need is more themers to give MythUI a good workout, it's an order of magnitude more capable than the previous UI. Try to remember what it looked like back in 0.20.
The switch to OpenGL for video rendering is about far more than the OSD. It's not least about code simplification, allowing us to focus on a single powerful cross-platform solution. It will also allow us to integrate the UI and video so that they are no longer two completely separate modes but a seamless experience. With OpenGL we can easily allow users to see video previews integrated into the list of recordings and video, browsing of rss feeds whilst video plays in the background and video continue to play smoothly as it's resized from a small preview window to fullscreen. These examples are limited by my imagination and my ability to communicate the benefits. If you can't understand what I'm trying to convey then you will just have to trust me when I say that is well worth sacrificing support for the oldest generations of hardware.
These screenshots might help to illustrate just one of the reasons we don't like X-Video:
First the OSD on a standard definition recording with VDPAU -
http://miffteevee.co.uk/imagebin/osd_font_vdpau.png
Now that same OSD with the same recording using XVideo -
http://miffteevee.co.uk/imagebin/osd_font_xv2.png
The above is a generous example, full resolution PAL, the lower the original video resolution the poorer the quality of the OSD so it's worse with NTSC/SD-ATSC and completely useless with videos/recordings below those resolutions. Also worth noting is that the OSD will match the video dimensions with Xv, so a 4:3 SD recording on a 16:9 screen the OSD will be squashed horizontally, vertically for a 2.35:1 film on the same screen and you can imagine how 2.35:1 on a 4:3 screen might be unusable.
We know not everyone cares about a better looking and more capable user interface. It's been made quite clear to us that it's what most users want. To be brutally honest it's also what interests the developers, which is usually a significant factor in a volunteer run project.
I'll finish by repeating that this is a long way off in the future. There is every reason to believe that drivers for existing ATi and Intel hardware will have much improved 2D OpenGL support by that time. Hardware has been capable of it for years, were we not playing 3D computer games using OpenGL ten years ago?