Beauty is in subtle details:

Sometimes a lot of people tend to forget in their textures are subtle details. These are things you don't notice at first glance, but make a texture or object interesting to look at.

Depending on the style and theme you can use a minimalist style, or go to extremes with the detail. It is your job to give the viewer something interesting to look at. Subtle details are perfect for this, and they can be everything. 
A sticker, some worn paint, rivets, bolts, someone that wrote something with a black marker, mud, oil, anything. But the key is to keep it subtle. If you go overboard with this, it'll lose it's power.

check out this link for some stunning artworks:http://stefan-morrell.cgsociety.org/gallery/259370