It's also a certain attention to detail with regard to things like use of whitespace, the layout and flow of widgets, the types of widgets used, and user experience in general. These things have become increasingly phoned-in, with little thought applied; just dump it all into a grid and apply copious padding and you're done. Things that don't fit get removed or buried in dialog tunnels and there is no effort to make anything clear to the user.

There have been flat designs in the past that don't suffer these issues, like the earlier designs of Mac OS (System 1.0-7.5), which worked fine on 1-bit and grayscale displays but had a lot of thought and research put into their user experience.

This all got thrown out in the transition to flat design. Not only was skeumorphism discarded, but so was care for UI/UX in general. The baby was thrown out with the bathwater.