Microsoft Office XML
4/15/2005 (updated 4/20/05)
Microsoft recently released (under license) the documentation for the new XML formats used by Microsoft Office 2003. This is why I think there has not been much uptake on it so far.
Dead Zone: most developers will fall into one of two camps: either they care very deeply about how the documents look, or they just want them be nicely formatted. Adobe PDF is a much better solution for the former. HTML is a much simpler solution for the later. MicrosoftWordprocessingML hits the dead zone right between these two uses.
Ubiquity: There is no browser support for it (which is probably a good thing, since it is a potent virus carrier). PDF and HTML are supported by virtually everything. There are ton of quality tools to view, process and generate them. Many of these tools are free and/or open. If you want to be sure someone can read it, MSWPML is just not a good choice.
Office 2003: Older versions of Office don't support it, so customers have to be on the latest and greatest version to use it. Microsoft is running out of compelling reasons to upgrade to the latest version of Office, so this isn't likely to change.
Expensive Edition: one thing that Microsoft is very quiet about is that the XML formats can only be saved with the more expensive versions of Microsoft Office. Normally, charging extra for such a useful feature would be fine. However, Microsoft should not be saying that "Microsoft Office" supports XML, but rather that only "Microsoft Office Expensive Edition" does.
Licensing - The initial licensing terms were pretty unappealing. Microsoft seems to have addressed the major issues, but it has taken a long time. Their continued attempts to patent it don't help either.
SlashDot coverage - not unbiased, but just because you're paranoid doesn't mean they aren't after you.