Clojure still uses S expressions, it just has more than just lists available. Square brackets don't make lists, they make vectors. Commas are always optional. I don't know what you mean by "arbitrary lists where pairs which belong together aren't grouped", but maybe you mean maps or sets, which are also not lists but a different data type.
The example that most typifies my issues with Clojure syntax is let-binding. In Lisp it's very clearly structured:
(let ((foo bar)
baz
(quux frob))
...)
In Clojure not only are the bindings unstructured, they are thrown into a vector. Not a list, not a map (for which Clojure has dedicated syntax, mind), a vector.
(let [foo bar baz quux frob]
...)
And that is one of the only two places I know of where Clojure arbitrarily uses a vector when it uses lists everywhere else:
