@Vish

>No one is expected to be an artist here, I'm just questioning why this is a bug!

I think everybody here understands that something like this isn't an actual bug. 
It's also true that what's pretty can be highly subjective. But that doesn't mean it's distributed equally. If we have a poll about every wallpaper out there, you'll see that there will be a strong general consensus about most of them. 

But you wanted some actual arguments about the wallpaper:

1. Two bright colors on the opposite side of the color spectrum are considering to clash according to color theory.

So, soft orange can be mixed with soft purple, but not bright orange with bright purple.
A good way to find colors that actually match is by using a tool called Agave, available in the Ubuntu repository.

2. This wallpaper has misdirection associations

Originally (in Lucid) it looked like a shiny purple surface. Which provided a nice strong contrast with the panels and the windows. 

But now that faint association is gone. It no longer resembles a shiny surface, because of the non shiny orange spots. 
People are now having faint association with a zoomed in picture of something biological. Combined with the clashing colors, they associate it with a decease. This is I think a bad thing.

3. Because of the lucid wallpaper that was the basis, people will focus on the transition, which makes it look sloppy

So Maverick is just like Lucid, but with orange spots? We visually separate the spots from the purple surface, because we know the old wallpaper so well. This makes Maverick look very sloppy, because the spots by themselves are very easy to make, and are judged by themselves.

So, what are the properties a good default wall-paper should have?

1. it should be easily distinguishable from the icons on top of it (people put files on their desktop!)

  - it shouldn't use any colors that used a lot in icons or the label underneath the icon  
  - it shouldn't use any colors that appear the same to a colorblind person as colors used in the icons or the label  
  - it should be easy to separate the 2D icon grid from the wallpaper in your mind. (in other words, it should be easy to tell for every pixel on the screen whether it belongs to an icon or the wallpaper). Possible ways to do that are:

      - use a 3D wallpaper
      - use something that has a strong contrast in color
      - use something that has a strong contrast in texture

2. people should generally find it pleasant to look at

    - the color should not clash. (clashing colors can easily be lookup, or you ask any person that consciously dresses themselves in the morning, rather than putting something on randomly)
 
   - it should have a positive association. If you take a look in a poster store, you'll see a lot of posters of animals, nature, landscapes, people and history rich objects. This seems to suggest those types of posters sell better, because a larger group of people find them pleasant to look at. A quick survery of most popular wallpapers in the community contest as well as the most popular wallpapers on gnome-look suggest these assumptions to be true.

To summarize:

 - the current wallpaper is bad for usability
 - the current wallpaper can be considered ugly on objective grounds 

People aren't complaining just because they don't love the wallpaper. 
It's because they distaste the wallpaper.
The difference is crucial!

It's impossible for a distrobution to have a wallpaper everybody likes.
But it's quite a feat to make a wallpaper that people actively dislike.

So let's treat this as a true bug-report. 
The current wallpaper upsets people.

Suggested fix: remove wallpaper, and default to single solid background color.
If the Ubuntu design team doesn't want that, they could pick one of the community wallpapers, which have proven that a large group of people is favorable of them.