If you look at the second picture on blenderguru you can see the reflected light that bounces around the support beams from the sun lamp (I assume)… I can’t seem to figure out where I messed up with that, mine are completely black with no ambient light, even though the sun lamp is illuminating everything else. I followed the tutorial to a T, and went back to the lighting segment (around minute 10 of the video) several times to make sure I didn’t miss something. The only thing I can think of is that the render options were maybe configured differently? Also I don’t know whether the boxy blue texture pattern seen on the top of the bridge has anything to do with it… doesn’t seem right either.
It’s been a while since I’ve watched that tutorial, but chances are he’s either using some slightly white/blue environment lighting, or is using the Cycles render engine, or has Ambient Occlusion turned on and set to “add” mode.
I tried that, it helped a little, but it didn’t really make the white parts of the bridge really ‘pop’ like Andrew’s. When I cut off all the ambient and environmental lighting, I still had that nasty jagged square texture thing going on on the image: