Why my Galaxy Nexus display is unable to present solid colors? Whenever there is a non-black solid color block in any application I can see tiny "watermarks", a pattern, and I don't even need to get close to see that.

The effect is more visible in lighter colors. The dark spots have more than 2 pixels in each dimension and they appear to be blue-ish.

Is it because of the Pen-Tile LCD configuration? Is my device defective?
It's likely the matrix which you are seeing.

http://en.wikipedia.org/wiki/PenTile_matrix_family

The Galaxy Nexus uses the RGBG matrix just like previous galaxy, however it's much higher resolution than many other pentile displays.

Here's a magnification of another, similar screen from a different samsung phone. http://www.droid-life.com/2011/09/28/samsungs-super-amoled-hd-screens-bring-back-the-pentile-matrix/

If that's not what you're seeing then it may be a hardware issue.