There was no public discussion. (Yes there were sessions on Unity at UDS, but these were about the path forward for Unity, not about whether it should be default or not.)
Making Unity was a decision made by Canonical on behalf of the Ubuntu community. It was announced in the first session at UDS by Mark Shuttleworth. As far as I'm aware this was the first time the idea was announced (although there were blueprints on Launchpad prior to the announcement that suggested it was a possibility).
All members of the Ubuntu Technical Board are Canonical employees (at least they all have @canonical.com emails) and I am sure they were all involved in the decision to use Unity.
Some things I think we should appreciate: it would have been very difficult for the community alone to make a unanimous balanced decision about the best way forward. This is especially true given Canonical's planned improvements to Unity in the next six months. Canonical was able to conduct usability testing, etc and also talk to OEMs (e.g. Dell) about the impact of the switch.
This being said, it can certainly be argued that the roles of the Ubuntu technical board, other Ubuntu teams and Canonical could made clearer.
I haven't done any development in Qt, but my gloss on the subject of accessibility there is that the concern might be a bit overblown. My understanding is that while GTK has built-in ATK support, Qt has to use a 'bridge' component to tie its native system to the accepted Linux accessibility framework.
Update -- I might have been premature in calling this issue overblown. For instance, as user9237 says, it doesn't look like AT-SPI was really ever implemented. So the trolltech doc quoted looks to be a bit "optimistic" .
Here is an interesting blog post, unfortunately still a bit old, covering a lot of these issues : Qt/KDE and the state of free accessibility.
Here's Mark Doffman's code site for Qt AT-SPI. Of which he says,
This project is a Qt plugin that
bridges the QAccessible API’s to the
AT-SPI 2 protocol enabling Qt
applications to be used with Orca,
Accerciser, and GOK.
The possibly bogus Trolltech/Nokia link:
Here's what Qt docs say about Qt
accessibility (from
trolltech/nokia):
Accessibility support in Qt consists of a generic interface, implemented
for a technology >on each platform:
MSAA on Windows, Mac OS X
accessibility on the Mac, and Unix/X11
AT-SPI >on Linux. Qt's accessibility
interface closely follows the MSAA
(Microsoft Active >Accessibility)
standard, which most clients support.
Other technologies used by Qt >provide
similar functionality.*
A slightly newer document gives an overview of Qt accessibility.
By contrast, here's a quick run-down on accessibility interfaces from the Gnome project:
GAIL (GNOME Accessibility Implementation Library) is an implementation of the accessibility interfaces defined by ATK. GTK is a toolkit which is already mapped to ATK by the GAIL module. License, download and other information can be found here. The GAIL source code also serves as an excellent tutorial for advanced ATK usage. In addition, you may be interested in the GAIL Reference Manual.
Best Answer
Nux wasn't exactly chosen - it was created from scratch and is developed alongside Unity. For some information on what it is and the benefits see this posting from Unity developer Jason Smith. Also there is a question that goes into much more detail on the inner workings of Nux.
Qt has recently been praised by Canonical's Matt Zimmerman on his blog for being stable on ARM platforms as well as x86; for being cross platform; and for having a mature touch input system.
I know this doesn't completely answer your question but it gives some insight into reasoning behind these decisions.