It is difficult to achieve elegance, efficiency and parallelism simultaneously in functional programs that manipulate large data structures. We demonstrate this through careful analysis of program examples using three common functional data-structuring approaches--- lists using Cons and arrays using Update (both fine-grained operators), and arrays using make array (a "bulk" operator). We then present I-structures as an alternative, and show elegant, efficient and parallel solutions for the program examples in Id, a language with I-structures. The parallelism in Id is made precise by means of an operational semantics for Id as a parallel reduction system. I-structures make the language nonfunctional, but do not lose determinacy. Finally,weshowthateven in the context of purely functional languages, I-structures are invaluable for implementing functional data abstractions.