What I found weird in Lisp (and didn't even realize at first) is that
foo
and
(foo)
mean something different.
I now understand it similarly to the way in set theory x and {x} are different, but one is not used to the ordinary parenthesis symbol behaving in this way.
They're fine with:
And stuck at: I personally don't mind s-expr syntax, but not having any infix expressions or precedence levels means a lot more parentheses in idiomatic code.