For me it's the fact that Kazunori is really just trying to recreate the joy of driving these cars, and when it boils down to it that means the physics have to be accurate, the graphics have to be jaw dropping and the cars and tracks have to be accurate. He's basically allowing us all to do things that most of us can only dream of ever having the chance to do.
Gran Turismo 5 shows us where the GT series is headed in the future.. accurately created weather effects, more real life racing series' being officially included, more real life tracks, and every form of car imaginable, old and new.
I am so so glad that GT is headed towards realism, and is always aspiring to be bigger and better. GT has always been the best, no half measure, no half hearted gaming, no money grabbing relentless yearly updates. They care, they really care, and I cannot wait to see what GT6 or 7 will be like. See you in 2020