Plastic does crack - thicker, heavier, higher grade plastic would be equally as prone to cracking. The point is, only a small percentage of iPhones sold do crack, and they are immediately replaced under warrantee. The quality of the plastic and components of the iPhone are pretty high, by most standards.
I once leased a Rolls Royce for a summer. The car didn't turn on - due to a loose starter cable. I suppose the overall quality of the RR ought to be reduced because of this.
So your overheating issue has been fixed, yet it remains a quality issue? I suppose some esteem quality to mean free of all glitches. Had your examples stated that the button on your iPhone popped out, the glass screen cracked, the eMac's housing fell apart, the disc tray snapped off, the keyboard of the MBP collapsed, and the unit creaked and rattled each time you lifted it, and all of your units came to a grinding halt, I would tend to agree that you had issues with quality.