I think that the syntax has some hurdles, but it's not the parentheses: It's to mentally understand when lists and symbols are data and when they are code. That's a problem not found in other programming languages and at the same time it is an interesting feature. Lisp is not alone to have such hurdles - another example would be Haskell which is also more difficult to learn than the average programming language (lazy evaluation, type system, monads, ...).

Often Lisp had been used as a teaching language for computer science concepts (recursion, evaluation models, algorithms, etc.) and thus it was associated by students with novel concepts they struggle with and not with solving practical problems. A typical example is the SICP book. It's great, but mostly CS and mathematics oriented -> the result is that the feedback is mixed.