I'm experiencing a similar problem to this thread in my personal machine (pentium 4 3GHz, 2GB ram, GeForce 6200 using nvidia-304).
After the update to Kubuntu 15.04, the desktop showed nothing but a black background with the mouse pointer, no response to ctr+alt+supr, but it was possible to login in another terminal with ctrl+alt+F1. And there end the similarities with the aforementioned link, because even after reboot the frozen desktop remained.
After retrying the install without the nvidia proprietary driver, the desktop showed, and it was usable after disabling compositing (Nouveau has the 'virtue' to corrupt many widgets in my machine). Leaving the compositing disabled allowed me to install the nvidia-304 and get into the desktop without hassle.
But here begins my problem: the performance of my graphics are quite poor compared to what I was accustomed in 14.14 and even some OpenGL aplications like glmark2 show error messages like
Inconsistency detected by ld.so: dl-version.c: 224: _dl_check_map_versions: Assertion `needed != ((void *)0)' failed!
Applies that glmark runs with Nouveau without error messages, but it does with a disappointing performance.
I'm suspecting of a kwin issue, but I can't seem to find nothing of relevance beyond pre-release issues.
Anyone knows of a definitive solution besides returning to 14.10?
Thanks for your help
EDIT: This is not a driver install issue at all, because the driver is the one suggested by kubuntu and the last one supported by Nvidia.
Given that my card is just a old AGP card (a GeForce 6200 as mentioned above) it is unsupported by the newest drivers, and because the card worked fine with the same driver with all the previous kubuntu versions until 14.10, desktop compositing included, I still expecting the same behaviour for this release with the same driver version.
The only things that I think could break the setup are wayland or a buggy kwin, because of the locked desktop screen after the update (driver 304 running with compositing enabled in 14.10)