That thing looks MESSED UP! What's also interesting (well, to me at least) is how brown everything got tinged- I guess that makes sense from the fact that the orbiter was essentially cooking like a giant plane-shaped marshmallow every time she came into atmosphere.
Most interesting to me is that there's a combination of tints, streaking, and just plain dirt- it's very much non-uniform. "She may not look like much but she's got it where it counts" 
I'm looking forward to applying this to my gray space triangles of doom!
It's realistic and I didn't even realize it!