For some lectures on e-document formats, I was looking to do a comparison of the technical features of the ODF and OOXML document formats. I came across an ODF/OOXML technical white paper by Edward Macnaghten. This is a useful comparison with good detailed analysis using the same documents marked up in the two formats.
Macnaghten points out that ODF makes more use of existing XML and other international standards, whereas OOXML tried to maintain compatibility with Microsoft Office features. An example of this is that ODF uses ISO standard format for dates, whereas OOXML uses an quirky Microsoft format. He also points out that ODF is more verbose than OOXML with its markup, but this makes for a more standard logical structure and the size of the file will be taken care of by the compression used by both formats.
However, I don't agree with the introduction which claims ODF was developed as a vendor neutral standard, whereas OOXML was by Microsoft. As explained later, ODF was derived from the format of Sun's StarOffice product. The details of how this was done are more visible than Microsoft's development of OOXML. But both ODF and OOXML were adapted from existing office product formats.
Both ODF and OOXML suffer from a lack of compatibility with the common web format: HTML. You can't simply open a word processing or presentation document in a web browser, it has to be converted in some way first. The obvious way to do this would be to base the word processing and presentation formats on XHTML. That way the documents could be made compatible with web browsers. It would be much more difficult to use XHTML for the spreadsheet format and there isn't a really good reason to do that.