Look at an Amiga. Find out how Linux is not like an Amiga, UI-wise. That's probably what's wrong with it, from Sassenrath's POV.
Glaring error #1: At Commodore it was a rule that user input should be prioritized over all other processing. This determined things like, for example, how interrupts were prioritized in the kernel. No modern OS does this, resulting in UIs that lag and shudder as the machine restores some process out of swap before getting around to the thing for which it was built: responding to your commands.
Yeah, some examples would be nice. I haven't used Ubuntu in a while but when I did I found the UX pretty good. My wife used it, really didn't notice it was linux, just wanted to know where the web browser was. Since then I've moved on to Windows 7. I should install Ubuntu again to compare.
Glaring error #1: At Commodore it was a rule that user input should be prioritized over all other processing. This determined things like, for example, how interrupts were prioritized in the kernel. No modern OS does this, resulting in UIs that lag and shudder as the machine restores some process out of swap before getting around to the thing for which it was built: responding to your commands.