Obviously QC is important. But don't forget, the more you make, the more defective units will slip by. This doesn't neccessarily mean a disproportionate number of defective units. Last time I checked, there weren't too many tablet makers, or netbook makers, who have sold in the millions of units of one model. If they did, you would be reading about defects.
Also remember that people don't generlly post about the defects of non-Apple netbooks on a Mac forum. Not sure where your "plenty of" data comes from.
While the backlight bleed might seem like a big deal here, it's one of the few defects I've seen or read about here that hasn't made it to mainstream media (yet). And believe me, they're ready to pounce on Apple for the slightest bit of news, good or bad.
Apple has probably sold at least a million iPad 2s by now. If 5000 people complain online about backlight bleed (and here on MR that number is less than 500), that equates to .5% defective units. I haven't seen 5000 complaints on Apple's discussion boards either. Even if you added up all the online complaints, with all the offline complaints, what are we talking about? 10,000? 20,000? 50,000? 50,000 is still only 5%.