Reading your bugreport I assume you didn't upgrade using "update-manager -c -d" but you used apt-get? The recommended way for gui users is using upgdate-manager because it will avoid this kind of problem.
To get your system back, try
$ sudo apt-get install ubuntu-desktop^
and let me know if it brings back the system for you.
Thanks for your bugreport.
Reading your bugreport I assume you didn't upgrade using "update-manager -c -d" but you used apt-get? The recommended way for gui users is using upgdate-manager because it will avoid this kind of problem.
To get your system back, try
$ sudo apt-get install ubuntu-desktop^
and let me know if it brings back the system for you.
Cheers,
Michael