Brilliantly written 'feel-bad' fic.
A respectful reconstruction tries to put the pieces back together and make something novel and amazing with them.
This does not bother with that aspect.
Still! For an attempt to take a happy story and turn it into a depressing one, this is probably the definitive example.
It's well written. It explores its concept well. It's about making you depressed, and it pulls that off!
(It is, admittedly, pretty much jossed by later books (or even more depressing, depending on your take.))
If there's any flaws, it's actually a surprisingly old complaint:
This presentation of Haruhi's character can't grow.
Worth a read, but very much a downer.