It annoys me how poor the manufacturing process of LCDs monitors has become. Now that dead pixels are finally few and far between, we have even worse problems such as with the backlight. It should simply not be necessary to have to go through 2 or 3 displays until you get one you're satisfied with.
Apple's screens vary from one to the other, some being better than others. It's unlikely you'll get one that is 'perfect', but perhaps if you exchange the iMac you might get one you can live with.