The spec for .DOC is not open. It has been reverse engineered with much trial, error, and gnashing of teeth. That's why there are so many formatting issues with .doc files in OpenOffice if they have any complexity to speak of. Most of the time, it does a pretty good job, but you can get some serious weirdness if someone uses footnotes, endnotes, versioning and other similar stuff. Quite frankly, it's amazing it does as good as it does, but it's not perfect (yet).
The OpenOffice format OTOH, is completely open in all respects. Apparenly the AbiWord and OpenOffice folks have gotten together over some stuff, and the upcoming versions of both word processors use the exact same file format. One thing I like about OO's file format is that it is Zipped and the actual formatting information is separate from the content. I've seen some neat stuff done with batchfiles manipulating .sxw files to do search/replace and formatting changes for mass edits on the fly.
That is a useful feature IMO. Let us say you work for for ABC Widget Co., who just had a merger with Bongo's Knicknacs Inc. You have about 100 documents that you need to change to reflect this change. A little knowlege of the format would allow you to create a very simple shell script that could be executed once in any directory containing the OO files you want to change that pretty much does a global search/replace, and poof you are done.
Granted, your average secretary probably couldn't write the script, but someone else could write it and distribute it to anyone who needs it.
And a cvs diff is useful..