It is difficult simulteneously to achieve elegance, efficiency and parallelism 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, defining precisely the parallel operational semantics of Id, a language with I-structures. We show elegant, efficient and parallel solutions for the program examples in Id. I-structures make the language non-functional, but do not raise determinancy issues. Finally, we show that even in the context of purely functional languages, I-structures are invaluable for implementing functional data abstractions.