Ubuntu is, in my opinion, one of the easiest desktop Linux distributions around. It caters directly to the end user, and recently such initiatives as the "100 Papercuts" project show how much Canonical is dedicated to improving usability in future versions.
However, one of the complaints that I see come up in the community the most is the fact that although Ubuntu is now competing directly with Windows 7 and Mac OSX, it has lacked that extra visual spice to make Free Software all the more appealing to end users. However, some of the new mockups and goals are extremely promising.
With the release of Ubuntu 9.04, codenamed "Jaunty Jackalope", the Ayatana desktop initiative was launched at creating various usability fixes in Gnome. The most visual of changes was the inclusion of a new notification system. Instant Messengers, IRC clients, mail notifications, device notifications, and all other system notifications were unified under the improved libnotify framework.
One of the central points of focus is a new boot-up experience for the next release of Ubuntu, codenamed "Karmic Koala". Usplash is apparently being replaced with GRUB2. The new splash process seems to be based on Grub2 and could incorporate smooth, subtle animations.
On the desktop side of things, scores of concept themes and icon concepts are being looked at right now by the Ubuntu Art Team.
The Breathe Icon Set
Alongside that, the Artwork Team recently started a contest initiative for user-submitted artwork to be included by default in the upcoming version. Many of them are absolutely stunning.
Okay, perhaps I went a little overboard on wallpapers. But the truth is, the community has contributed a lot of amazing works...over 900, in fact.
Karmic is shaping up to be an extremely promising release. This may be the strongest push for improvements in Ubuntu I've ever seen before.