That is a great question and one that even professors of literature debate. I'd say that these are elements of writing that should receive some feedback:
Purpose: A writer's purpose should be a primary consideration. What did he hope to accomplish with this piece? How effectively is that message conveyed? Does the form support this purpose? Consider a student who has written a poem about death. Does her use of iambic pentameter support her intended purpose, or does it detract from it? Does the use of free verse seem elementary, or is her purpose strengthened because of that choice?
Clarity: Where are the holes? Even authors who make a substantial living through writing creatively depend on their editors to find the "holes" in their plot, character development, timelines, and uses of flashback and flash forwards. After reading the piece, where do you feel more clarity is needed?
Voice and Style: Does the author make an intentional focus to use strong verbs and descriptors that bring the piece to life? Does it feel like a flat and one-dimensional effort, or do the characters feel fresh and new? Does it seem like this author has crafted a unique style all her own through this piece? If not, where do the stylistic techniques fall flat? Did he include literary and poetic devices to further the purpose? If he utilized devices like metaphors and hyperbole, do these feel trite and cliche, or are the devices crafted in new and interesting ways?
Grammar: Lots of people don't like to talk about grammar usage, but the truth is that messy grammar can impede comprehension. Does the author craft sentences of varying sentence structures for interest, or does a long list of simple sentences fall flat? Are commas well-placed, or are they missing almost completely?
I hope this helps you tackle a few ways to approach critiquing a piece of creative writing. Good luck!