It doesn't work as a user-friendly desktop OS. I've used Linux for a long time, I'm not a novice user. I put Ubuntu in a VM and it installed fine, everything worked great. Then I went to delete a folder off the desktop by dragging it into the trash. It wouldn't let me. Didn't tell me why, didn't give me the ability to authenticate to delete it, nothing. I had to drop to the terminal to delete the file.
Would a new-to-Linux user know that he has to drop to terminal to delete a file sitting on his desktop? Who would expect that dragging a file from a CD onto the desktop then trying to delete it would require a sudo command to delete? Linux is not even remotely close to ready for the desktop.