Who is the narrator of the story and why is this an effective point of view?
This is a much debated question of the short story. The fact is, the narrator is left "unknown" on purpose. It is obviously a first person narrator - who speaks as if very familiar with the small town and the Griersons - however, her familiarity does not come from a friendship with Miss Emily, but rather from a distant sense of know-it-all-ness (if you will permit the word).
I like to think the narrator is a female - someone who has grown up in the town and been surrounded by gossip most of her life. She tells the story of the Grierson's with details many men would not think of including (see the first two paragraphs of the story again - and consider the way the house and Miss Emily are described - definitely a feminine perspective).
I think Faulkner does this for a few reasons: first, it makes the reader pity Miss Emily even more. She is pitiable because of her circumstances and certainly because of the choices she makes in her life, but on top of all that - the town gossips about her in a way that almost makes us want to take her side. I also think Faulkner intentionally leaves the narrator's full identity ambiguous because then it makes it more like "the town" vs. Miss Emily. This sets up an underlying external conflict that brings many more facets to what would otherwise be a fairly simple story of crazy-neighbor-goes-psycho, kills boyfriend, sleeps with dead body. The ambiguous narrator brings the idea that perhaps the entire town is a little weird - not just the Griersons.
We know very little about the narrator in "A Rose for Miss Emily."
We know he is a member of the town, as he refers to it as "our" town. He seems to be someone who is familiar with both the town's present and its past, as he is able to recount stories from Emily's younger days. Thankfully, he is someone who is observant and attentive to detail or we'd actually know very little about Miss Emily's tragic tale. He doesn't appear to have any strong feelings about Emily, as he both describes her in an unflattering way and offers reasonable explanations for her present condition.
I believe the narrator is more sympathetic than not, based on the kinds of details he shares with the readers; nevertheless, he is mostly a dispassionate (unemotional), disinterested (impartial) observer. That's effective because it allows the reader to take in all the facts and details, then make a personal judgment about the plight of Emily. We can choose to see her in a sympathetic light or a more sinister cast; the narrator simply shares with us what he sees.