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.