> Use 4 colors, fill it up with whitespace and cram everything into squares.
For me the worst of those is the prevalence of whitespace, and the loss of the grey background.
Grey was the greatest color for UI background, it wasn't overly bright, it won't burn your eyes like modern UIs that shine like a thousand suns, no need for separate "dark theme" because a grey theme is comfortable in all light conditions, it allowed the content color to go in two different directions (if your background is white then your content can only get darker, but if your background is grey your content can be darker or brighter than the background), and it has a clear visual separation between the grey app UI (menus, toolbars, ...) and the white content (E.G. document being edited).
For me the worst of those is the prevalence of whitespace, and the loss of the grey background.
Grey was the greatest color for UI background, it wasn't overly bright, it won't burn your eyes like modern UIs that shine like a thousand suns, no need for separate "dark theme" because a grey theme is comfortable in all light conditions, it allowed the content color to go in two different directions (if your background is white then your content can only get darker, but if your background is grey your content can be darker or brighter than the background), and it has a clear visual separation between the grey app UI (menus, toolbars, ...) and the white content (E.G. document being edited).