Playing games on 51D550 is insane, the colors are so rich and natural !! Also 51D550 is capable of led like brightness(approx 160 cd/m2) which no panasonic plasmas(60 cd/m2 for vt20) and even Samsungs own D8000 plasma(90cd/m2) is not capable of producing and is tailor made for games in 3D!! Also its got 18ms input lag!!
You are getting 160 cd/m2 because there is no anti-reflective filter limiting light output. This results in glare. You dont need 160 cd/m2 brightness in the night. During morning light, even with lower brightness, the sets with anti reflective filter produces better picture. Brightness can overcome reflections only so much. That is why high end plasmas come with filters.
Having said that, D550 is a great TV in its own right and its colour reproduction is one of the best (if not the best).