They fixed many issues with default user interface elements. For example, it's almost impossible to see if a control is disabled or not in Yosemite. Or the def

They fixed many issues with default user interface elements. For example, it's almost impossible to see if a control is disabled or not in Yosemite. Or the default, white, borderless buttons on don't work very well on a bright background. In El Capitan they fixed a lot of these problems. All buttons now have a border, and their state is clearly visible.