Pleasant looking application and work environment contributes a lot to the experience of using an OS (that's the main appeal for OsX, IMHO).
Ubuntu is for humans, humans likes pretty things.
Anecdotal evidence is the popularity of sites such as gnome-look, deviantart, and interfacelift, as well as the time and energy invested in configuring the visual side of the operation system (see the popularity of "post your desktop" threads).
Treat aesthetics and usability as features:
1. HIG violation as high priority bugs.
2. Missing Icons, cluttered interface, poor looks - medium priority bugs.
3. Theme, Icons, wallpaper, bootloader, application layout and consistency, and the rest of the visual interaction between the users and the machine should be high quality, scrutinized, voted, and submitted by both professionals and the community.
4. if possible, integrate a skin browser with gnome appearance manager