When people point to examples of bad brutalist architecture, I point them to the Barbican as a beautiful counter-example.
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A Brutalist building with zero plants looks like a totalitarian prison hellscape designed to destroy your soul before it destroys your body.
A Brutalist building surrounded by trees with every nook containing greenery and vines dangling down looks like some kind of idyllic Star Wars planet populated by fuzzy hobbit-like creatures.
I'm not sure why I find this effect so strong. Perhaps because flat gray concrete is aesthetically ambiguous. When paired with greenery, it looks like stone. In it's absence, it looks like industrial mechanism.