On my 3rd macbook now.
1st one had discolorations on the top of the keyboard which you couldn't wipe off. Store employees used a Mr. Clean pen which didn't work, so they swapped me a new one.
2nd one had a large scratch on the upper right part of the outside, so I had that replaced as well.
3rd one is doing pretty well so far, no outside marks or dead pixels.
Some of the things I noticed while I had 3 of the different macbooks were that there are definitely differences between the panels. The 2nd one had worse negatives and worse colors on the LCD. I forget what the exact panel model was, but it was different from the 1st and 3rd which are both 9C89, and look about the same.
On the 2nd one, I could also see clearly how the manufacturing wasn't smooth on that one. Inspecting it closely, there would be close to invisible bands along the aluminum where carvings were made. Also there were nicks and spots elsewhere on the case that look like dark tiny dots. The 1st also had a couple insignificant ones on the side that didn't bother me.
Lastly, this is going to be speculative, but I feel like the smoothness of the trackpads also varies among models. The 2nd was the smoothest, the 3rd feels rough. Not sure what to make of this though, but it could be just because of moisture on my fingers. So far it doesn't feel the same.
Hope this helps.
I must be going a little nuts as well, because on this 3rd macbook, I finally just saw the thin glowing white LED bar when the macbook sleeps.. Pretty sure my 1st one did not have this because I was looking for it and thought that apple had omitted it. Not a big deal, but nice to have it back on this 3rd one.