Good desktop wallpaper uses medium or dark colors. Light colors are painful to look at for a long time, and they distract from the lighter colors of the actual content you're looking at.
Good desktop wallpaper doesn't have distracting textures or objects along the left side or top; that's where my icons go, and I need to be able to read the text.
Good desktop wallpaper is at least as large as your monitor.
Good desktop wallpaper is the correct aspect ratio, or somewhat wider.
Good desktop wallpaper is composed like a good photograph, even if it's not a photograph. It shouldn't be muddled or overcrowded.
Good desktop wallpaper has a cheery disposition, rosy cheeks, and plays games—all sorts.
That's what I look for in wallpaper. Did I miss something?