An OS, no matter how technologically or ideologically advanced it is, should also be visually appealing.
And Ubuntu, being one of the most beatiful (I mean the default appearance here) distribution of Linux still can't compete with Vista (not to mention Mac OS X).
Here I imly the following:
-icons. All the icons should be converted into vector graphics, so that they were scalable (so that they could look the same everywhere).
-fonts. Even Liberation fonts look better than default GNOME's dejavu, but still they leave much to be desired.
-screensaver. Currently, it's just a black screen. Enough said, I guess. Ubuntu should offer aset of nice screensavers, basic, or OpenGL-driven.
-applets. GNOME applets should grow in numbers and functionality. At least I would suggest a wrapper that could display a Web widget as an applet, so that user could simply paste BB-codes from YouTube, Jaiku, Vimeo or else and see it on his desktop.
-3D effects. There shouldn't be a vast number of them, but all of them should be quality-driven. Just look how bad is Compiz rendering as you make anything that drives windows borders into curves - they are edgy and twisted.
-wallpapers. I know, Deviantart contest is on, but here is also another point: Ubuntu should provide numerous wallpapers out-of-box, and all of them should suit the default colour scheme. The same stays for login screen.
-themes. Ubuntu should provide themes that change the appearance more globally. Not only GNOME's appearance, but all the apps, bootloader, login screen, screensaver.
-customization. Changing Ubuntu's appearance isn't really easy these days, since you can't tune up all the aspects in one place. I mean, you can change the theme, but you should work really hard to make your style as sole as the defaut one - Qt, GTK and other (wxWindows, etc) applications share different configs. Just try to switch to a darker theme and you'll learn how many elements would require tuning. Ubuntu can offer great means of customization, but that doesn' mean that an average user is able to make a cool-looking style.
-bootloader. That was already mentioned, but still. Ubuntu should show text mode at all. Currently, it can occasionally fall back to console for a while, which is discomforting.
-hibernation/suspend screen. The same.
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