Because maybe he doesn't know what he's talking about. Personally having used the old Gmail interface and the new Gmail interface extensively, I think the criticisms do mostly boil down to change aversion.

Just to take one point: the icons are confusing at first, but mail is something you daily for years, and Gmail has always been optimized for power users, so even though it might take you a week or two to get used to the icons, it's a benefit because now a greater percentage of the words on the screen are actually your email content.