This is a hot topic in New Zealand right now, and an interesting relevant rant about OOXML technical issues just appeared on Slashdot, so I feel like getting my 2c in...
Rod Drury's blog has an interesting comment:
The antagonists seem to change between the spec being either
a) Too long and detailed for people to be able to implement or;
b) Too vague and lacking in specific detail for people to implement.
It's entirely possible for both of these to be true at the same time --- when the thing you're trying to specify is much too complicated, so that a specification with all the necessary details is much too large to be correctly implemented by anyone. That is exactly the situation OOXML is in; it wants to specify every detail of the behaviour of Microsoft Office, millions of lines of code accumulated over 20 years.
People are worried about the cost of migrating documents from Word format to ODF. I don't think anyone should be advocating such a migration; legacy documents, if converted at all, should simply be printed to PDF for archival. The real issue is what format new documents should be produced in. OOXML is clearly a horrible format from a purely technical point of view --- not even its defenders seem to be challenging this --- so there's a strong argument that you do not want to be producing new documents in OOXML if you have a choice. I think having OOXML stamped by ISO as a de jure standard would send the wrong message about that. On the other hand, making it an standard does not help at all with the problem of preserving legacy documents.
The overall most cost-effective and future-proof solution IMHO is for ODF to be the single de jure office document standard, for Microsoft to ship ODF read/write capability in its products, and for people who care about the longevity and openness of their documents to make ODF the default for new documents, while continuing to work on old documents in their existing formats. Presumably Microsoft opposes this approach because it would ever-so-slowly weaken people's dependence on their products.
Someone might wonder whether support for the WHATWG's efforts to standardize "Web-compatible" HTML is consistent with opposition to Microsoft's efforts to standardize "Word-compatible" OOXML. It is, because there are actually many differences. The main difference is that Ian and his gang aren't just specifying "whatever IE does"; they typically look at what a number of implementations do, and choose the behaviour that makes the most sense while remaining "Web compatible" (which is often, but certainly not always, what IE does). This is made easier because HTML and its related technologies originated in the standards world, whereas Microsoft's Office formats have always been wholly their own monstrous babies. (Another major difference, I suspect, is that conversion of HTML documents is a lot less feasible due to greater reliance on embedded scripting to give meaning to pages.)