> It had significant whitespace and some other Python-inspired idioms like array comprehensions and object comprehensions.

Modern JS has standard HoFs (on arrays) and an iteration protocol.

> undeclared vars leaking to global scope

Strict mode (which syntactic ES6 features enable by default e.g. class contents are in strict mode always).

> baroque keywords like "with"

Which can trivially be linted away.

> ES6 did borrow many good features from CS

Not really from CS, rather from similar (much older) sources CS got them from.