As a daily user of Ubuntu, I have few complaints. With that said, however, the X window system is… dated. It’s pretty much rock solid out of the box, is fairly well maintained, and has few issues in my experience.
Mark Shuttleworth of Canonical/Ubuntu sees X being replaced by a “younger” display server called Wayland, possibly as soon as Ubuntu 11.10.
Wayland is based on GL, which means the eye-candy we’re used to seeing in other OS’s will be easy to implement, out of the box, in Ubuntu.
More details here: