The following are problems with the OOXML specification which make it not an open standard. This page is a summary of the problems. For a complete list of the technical problems, see the list on Grokdoc.net. http://www.grokdoc.net/index.php/EOOXML_objections
It cannot be implemented
The specification relies on mimicing the behaviour of previous versions of Microsoft Office, but this behaviour is not publicly described anywhere.
Microsoft's patent promise http://www.microsoft.com/interop/osp/default.mspx does not protect people who make software compatible with Microsoft Office. http://www.groklaw.net/articlebasic.php?story=20080326151405938
Further concerns were raised by SFLC. http://www.softwarefreedom.org/resources/2008/osp-gpl.html
No one has read it
The original specification was over 6,000 pages. The national standards bodies submitted 11,000 problems in September 2007. In the Ballot Resolution Meeting in Geneva in February 2008, 900 changes to the specification were adopted to address these issues without discussion. It is highly unlikely, and maybe impossible, that anyone successfully applied all those changes and read the resulting text.