Having disassembled one as far as I could without crossing the point of no return, it's definitely a design that strives to be simple and easy to make. I can't think of any aspect I'd call deficient or flimsy. My guess is the squeal flaw is very simple-- from what I can see in this pair, the application of the conductive layer to the stator surfaces 'oversprayed'. That's just a smack-your-forehead manufacturing defect. It's not a problem that crops up, it happens straight out of the box.
Personally, I don't see the choice of ABS (and styrene internals) over aluminum and wood as being inappropriate. Acoustically, it's going to behave better and it's lighter. It's also much much less pretentious.
I agree that if Koss got their ducks in a row, this headphone could be sold for much less, and it doesn't have the same 'what the heck?' DIY-type finishing errors the hifiman cans tend to have... though to be fair to hifiman, I'm betting their return rates are lower ;-)
...and I meant it. If I could find a source of 1-2micron mylar, I'd make a go at reproducing these cans in my shop. Damn, now I really really want to buy Koss. I wonder how much I could get on Kickstarter...