I don't know much software for OS4 which is labelled as alpha/beta, most of the software released there are stable versions, and not all of the software for OS4 has a MOS version.
Not a problem since MPlayer exists for OS4 too, and it's free as well. From my experience, DVPlayer handle DVDs better than MPlayer.
Erm, MPlayer has more complete and faster support for DVD than DVPlayer (and don't blame the DVPlayer author, it's really quite some work to support it fully, even when reusing ffmpeg), and it even supports DVD menus (experimental, though). Whether the OS4 MPlayer port supports all this correctly is another matter.

That will be probably fixed with a future Timberwolf release with support for HTML5 videos. BTW MOS currently lacks a Firefox port.
Timberwolf already has HTML5 video support, but only for Theora, not H264, which is used in Youtube, Dailymotion, Vimeo and a couple others. However, google is pushing its VP8 codec. So, if FireFox 3.7 is ported to OS4, you'll get VP8 support, and only then, you'll have a chance to play youtube in timberwolf (except if you get flash first, but i seriously doubt it).