I often feel like I’m the only person who likes the sort of extreme skeuomorphism of Apple circa 2007: it wasn’t perfect, but it didn’t have the soulless corporate feeling of flat UI styles.
The problem with flat UIs is that they're also in the middle of abandoning any conventions on what's interactable as well.
Ironically Windows circa 3.1 and definitely by 95 had this nailed: is it greyed out? 3D? You can interact with it. Not 3D? You can't. 3D but greyed out? It is contextually disabled.
Simple and clear at a glance. What that interface got wrong was the MDI motif - multiple document interface never really worked as well as Microsoft wanted, although if they'd made the leap of making it tiling by default they would've got there.