Just switching to ODF as a default format wouldn't have been a good idea, the difference in data models would have caused problems. I have in mind that to do that we'd have to change the internal model for things like list and tables to not have to convert them back and forth each time with losses.
This is a sad fact of the reality.
BTW one of the glorious hackers from the "LibreOffice side" has implemented a LibroOffice filter to read AbiWord documents.
Abiword does have support for ODF, but it could be better. Abiword 3 uses ODF 1.1. At the last ODF plugfest quite some issues were found [1, end of page].
There are now automatic tests than can be run to improve ODF support. The tests can be run from the command-line and give output as HTML [2].
Currently, there is a project underway that makes it possible to run the tests on a public test server which will show live results.
hub asks for tests in his blog post. For loading, saving and rending ODF files, the ODF community can help.
[1] http://odfplugfest.org/2015-thehague/report.html [2] http://autotests.opendocumentformat.org/
There is a long list of bugs from every side.
Images are also PNG, JPEG and SVG. Formulas are MathML.
Styling uses the same properties as CSS, but the are written in XML instead of using the CSS syntax.
The ODF Essentials book is good introduction to the format. http://books.evc-cit.info/odbook/book.html
I'm personally not super happy about that, but it is still much better than not doing it.