Desktops succeed because they're consistent. Possibly the most important thing is to have a consistent API. X at this point is venerable. Nascent Linux desktops come complete with cruft. Lots of cruft.
Honestly I can understand why Canonical wants to start again (Wayland, Unity, etc). Even more honestly, I just don't see adoption of any Linux desktop going much above 1%.
I have a 6 core Xeon with 24GB of RAM on my desktop and Ubuntu still feels sluggish. Go down to 2 cores and 4GB of RAM and I'd have a better experience with either OSX or Win7.