On a colored titlebar, as you might find in a web browser for example, the new icons accomplish the feat of having at least one nearly invisible color no matter what color the titlebar is. And they don't have any fill to provide contrast, so the shape is lost. On my titlebars (darkish blue) the new gmail logo loses the blue and the green, so it looks like a dead worm.

The original gmail logo was awesome. Clean, recognizable, clever, easy to integrate. Everything about it was incredibly well designed. I'll miss it.