Have you ever thought about what makes your desktop look nice, and why many people may criticize, for example, Vista's UI? I decided to come up with a list of what seems to be that makes up a nice-looking desktop.
- Color Consistency - Take Ubuntu's default look. Brown. Orange. Pretty similar looks. It fits well.
Now, what if you take the default wallpaper but set the blue Clearlooks theme? It looks terrible, obviously - the colors aren't consistent. Remember that Ubuntu brown isn't necessarily the normal neutral brown - it's more of a dark orange. Now it's OS X's turn. The neutral dark grey used in OS X is what makes adding splashes of blue everywhere else acceptable.
- Cleanliness / Organization - The cleaner or more organized a desktop is, the more visually appealing it is. If you have various icons scattered over your desktop, it will look unbalanced. OS X's close, minimize, and zoom buttons are familiar-looking because most of us see a traffic light daily.
- Completeness -
nobody likes pixelation or squarish boxes for icons. For example, the Pidgin icon is an example of what an icon should _not_ be like because part of it is cut off.
EDIT: Now, be careful to not misunderstand me - I wasn't blaming the Linux UI (the Clearlooks/Ubuntu-wallpaper thing was an example of what somebody should NOT do), and Windows XP's UI isn't bad at all. The icon colors are generally consistent with the title bars, and I don't know how Microsoft managed to be able to get the XP-style (not pre-XP) icons to blend in to the interface so kudos to them for that!
Although many people say that a desktop looks should not matter, the look of our desktops may affect our productivity either for better or for worse.