Riding on the coattails of the 51GB HD DVD announcement was the approval of the dual-layer DVD / HD DVD combo disc format. These combo discs have been a feature of a several releases, but the linked article provides a good analysis of serious challenges the format faces. The bottom line is this -- for almost everyone, one of those sides goes largely unused. If you have a HD DVD player, the DVD side will only see duty in secondary applications. And if you don't have a HD DVD player, that HD side isn't going to see any use at all. And with the format war, average consumers don't value future compatibility. This wouldn't be a significant issue if there were no additional cost associated with the combo disc. But there is, and that kills the deal. There's plenty more to the critique, so check out the link if you're inclined.
EDIT: The 51GB HD DVD approval included the single sided combo format, not the dual-sided one currently used in several releases. - Thanks, Kevin!