Two words - cheaply made
I just set up a 55 with a friend.
First impressions are that this is one of the flimsier ,cheap feeling TVs I have set up in awhile.
Even the packing materials were several notches below typical samsung. The stand screws were not well fit, the whole case is thick and cheap plastic looking, the protective stryfoam only covered the front and left the back mostly uncovered.
That being said, it seems to have a decent picture out of the box in Movie mode. ( no adjusting yet.)
We did verify 4k with Netflix.
One surprise to us was that the tv DOES not support 3d passive or active. No big deal in this case, but that's probably part of the price differential.
My buddy is already thinking of paying the $400 difference for the 8550