Bright spots, color spots, shadows, text not that black and other problems. Go join MobileRead to find out more from actual owners who have sent them back multiple times for issues like those listed above.
Since I'm an actual owner, I'll take my own word on the Paperwhite's performance.
This is an image from mine in a dark room with the backlight set at 21. I got mine on November 4th, which is probably part of the second production batch.