Only just seen this. The main problem (in my experience) is the extensible segments of the .docx format, such that I'm not sure how much of this is good-faith 'improvements' and how much of it is MS trying to dominate the market by being silly-buggers.
It even causes problems with legitimate pre-.docx (also .xlsx, .dotx, etc) versions of Office. You can add a patch to make those readable, but it's not perfect. I know that using a pre-Office2007 version of Office might be considered a bit 'retro', for some, but despite the pressure from MS there's plenty of people who see no point splashing out the cash, especially when MS also completely ruined the user interface between 2003 and 2007 versions.
The official patch that lets (say) Office2003 read the XML-based documents reasonably well is usually good enough, even if it's read-only and you are forced to save to .doc/.xls/etc format if you edit anything. But it doesn't work perfectly. My advice is to try to persuade people to Save As .doc for you (or, actually, .rtf would often be both better and sufficient for most purposes).
Meanwhile the LibreOffice spin-off of OpenOffice (OpenOffice having stagnated since that fork-off, if anyone's still using that) is being improved to catch up with all the bells'n'whistles MS adds to its new file-type definitions. As are perhaps various other independent word processors with a decent development cycle behind them. LibreOffice will even read/write the "Office Open XML" formats while OpenOffice was only ever read-only, I think. i.e. If OO reads correctly (enough) you'll have to save as either .doc or .odt if you edit and change, but LO shouldn't quibble.
(Pro tip: If you have Open/LibreOffice, set it to save its documents by default in .doc format for the best 'popular' compatibility. Otherwise you're going to risk accidentally sending .odt files to an MSOffice-user (likewise .xls instead of .ods, etc.), which they probably can't open and end up asking you to send it again. Maybe the latest MSOffices read OpenDocument formats, but even if they do you're always going to find someone like my acquaintances who are 'happy' with their Office2003 who wouldn't be able to read them.)
And I'm reminded of the time when Word 6.0 (I think it was) had just come out and Microsoft released a patch to allow Word 5 to read Word 6 documents. There were carefully documented instructions on how to apply the patch... but in a Word6-format document that your Word 5 couldn't... before applying the patch... open!