For excellent coverage of the issue visit these two very well laid out posts by Greg Laden in 2007 when microsoft released the .docx file format. (Yes, I am a mac user and an Open Source software user. I do run a virtual machine with windows on it but use exclusively open source office software.)

• .docx is first and foremost proprietary. In principle, this defeats the collaborative nature of doing science and sharing work among colleagues
• .docx is NOT backwards compatible. Yes, microsoft made a program that is not compatible with older versions of itself
• In a blatant disregard for international standards, it proclaimed its “Office Open” XML (OOXML) code to be the standard. It is neither open, nor a standard by ISO
• It was rushed to acceptance by ECMA, where Microsoft staffed the chair
• In the face of the freely available open source OpenOffice software with its fully documented and  transparent ODF format, Microsoft has misused the meaning of  “open standard”
• Some file uploading programs do not recognize a 4 letter file format abbreviation, so uploading your .docx file may be shunned away by the error message of doom as an unsupported file type. This confuses collaborators and hampers progress
• XML is a wonderful, standard document format. Microsoft is to be applauded for moving to the open format, BUT instead going with the flow they actually had to proprietize something that is freely available and call it their own
• As such, OOXML is a broken format, with many errors and inefficiencies, it is defective by design. If you actually try to manually update the code correctly, the file cannot be read by Office 2007.
• Microsoft’s OOXML API will only install and run on Windows Vista and XP SP2, thus again stomping on the spirit of open source standards.
• Admittingly, I do not know the current status, but if you had developed a previous language library with previous versions of Office software, you are not allowed to keep that directory
• This is in part because Office 2007 doesn’t have a library to keep files in
• No support for 64-bit technologies, whether they are from microsoft or user-created
• Visual Basic-embedded macro do not work in their updated release for the Mac version (Office 2008)

Additionally, the following points are from the website <NO> OOXML, which has a petition where you sign up to protest against the the use of this broken attempt at a standard

1. There is already a standard ISO26300 named Open Document Format (ODF): a dual standard adds costs, uncertainty and confusion to industry, government and citizens;
2. There is no provable implementation of the OOXML specification: Microsoft Office 2007 produces a special version of OOXML, not a file format which complies with the OOXML specification;
3. There is information missing from the specification document, for example how to do a autoSpaceLikeWord95 or useWord97LineBreakRules;
4. More than 10% of the examples mentioned in the proposed standard do not validate as XML;
5. There is no guarantee that anybody can write software that fully or partially implements the OOXML specification without being liable to patent lawsuits or patent license fees by Microsoft;
6. This format conflicts with existing ISO standards, such as ISO 8601 (Representation of dates and times), ISO 639 (Codes for the Representation of Names and Languages) or ISO/IEC 10118-3 (cryptographic hash);
7. There is a bug in the spreadsheet file format which forbids any date before the year 1900: such bugs affect the OOXML specification as well as software applications like Microsoft Excel 2000, XP, 2003 and 2007.
8. This standard proposal was not created by bringing together the experience and expertise of all interested parties (such as the producers, sellers, buyers, users and regulators), but by Microsoft alone.

Anyone have more to add to the list?