Yeh the game looks very impressive. But some people were settings themselves up for disappointment with unrealistic expectations, the gameplay was never going to look the same as that cutscene
I think that it's just that a lot of people were saying it will look exactly like that first trailer and it set many people's expectations too high. I knew that it wouldn't look as good as that trailer if only because of the difference in animation between a cutscene and actual gameplay.
Nevertheless, it's an absolutely stunning game and I can't wait to play it. Uncharted 4 was one of the biggest things that drove me to get a PS4 at launch...just knowing that it was on its way.