A more elaborate email from Canonical's Rick Spencer has now hit the Ubuntu development mailing list that further analyzes the situation and discussion that came out of the Ubuntu Technical Board meeting.
The arguments for defaulting to classic GNOME in Ubuntu 11.04 come down to key feature regressions (such as no system tray support for many key applications), usability problems, and too many bugs on some hardware and just the final product being too buggy. Some of the cited usability problems are the settings are hard to find, the categorized view of applications, and the Unity search sometimes yielding bogus search results.
Though Canonical is pressing hard to keep Unity the default in Ubuntu 11.04. "Given the current course of development, it appears that we are going to achieve this goal, and Unity will stay the default for 11.04."
Additionally, "Representing the desktop team, Jason Warner believes that Unity will deliver the superior experience for most users in 11.04. I agree with this position and support staying the course."
But you can now hash it out on the Ubuntu development list if you would like to see the classic desktop used. Worth noting is that if this change happens it would be effectively to a GNOME 2.32 experience and not the GNOME 3.0 Shell, but both will be available regardless from the package repository.