Hum. Difficult. It's easier to say what I don't like.

I don't like walls that are too "dark" (not in color but in mood), or too weird. I don't like walls that look 3D. On the other hand, I'm struck in awe when I see something I think is a picture but actually is 3D rendered.
As a general rule, I like walls that are uplifting, not depressing. Thats why I usually don't like SciFi or Techie walls. To me, a wallpaper also can't have too much dark/light contrasts, or I loose my icons on my desktop (yes, my desktop is full of files and shortcuts).
I must admit, not too many walls fit my tastes, here or elsewhere.
Walls that I like are usually 2D abstracts, more not etherreal either. They are usually relatively simple too, and breathe a lot (not too tight of a design).
So, BoXXi, could you have put that as a choice?