1. The way that Holmes comes across the goose. Of course he doesn't go to the store and buy one. No, the goose appears slung across the back of a mysterious figure in the streets of London on Christmas Eve. The man with the goose is attacked, Holmes' policeman friend Peterson tries to stop the attack, and everyone but the officer runs away, leaving the goose in the street. Naturally Peterson takes the goose to Holmes to see what he makes of the incident, and Holmes concludes that Peterson should eat the goose himself. Would I eat something for my Christmas dinner that had been dropped in the street under mysterious circumstances? Probably not. But obviously it's a good thing that Peterson does roast the goose, because that's how he discovers that hidden inside the goose is an extremely rare jewel, the blue carbuncle.
2. The blue carbuncle. What is it? I spent way too much time thinking about this question. According to Holmes, it's "more than a precious stone. It is the precious stone." At first I thought it was a diamond, because Peterson tells Holmes that it can cut glass. It's "brilliantly scintillating," and diamonds can be blue, or at least bluish.
(For example, the Hope Diamond is blue.)
But Holmes says that the stone is "remarkable in having every characteristic of the carbuncle, save that it is blue in shade instead of ruby red." Carbuncles are red jewels, either garnets or rubies. But this can't be a ruby, because blue rubies are sapphires, which are lovely but not particularly remarkable. The blue carbuncle has to be completely fictional. Then I discovered, though, that blue garnets do exist. They were discovered in Madagascar in the 1990s. They won't cut glass, and they aren't worth as much as Holmes' blue carbuncle, but they are bright blue, sparkly, and very rare.
(You can buy your own blue carbuncle here.)
So now I think that Conan Doyle was prescient. The jewel in "The Adventure of the Blue Carbuncle" wouldn't be discovered for a hundred more years.
4. The opening scene, where Watson discovers that Holmes has been spending the post-Christmas holiday dissecting a hat:
"I had called upon my friend Sherlock Holmes upon the second morning after Christmas, with the intention of wishing him the compliments of the season. He was lounging upon the sofa in a purple dressing-gown, a pipe-rack within his reach upon the right, and a pile of crumpled morning papers, evidently newly studied, near at hand. Beside the couch was a wooden chair, and on the angle of the back lay a very seedy and disreputable hard-felt hat, much the worse for wear, and cracked in several places. A lens and a forceps lying upon the seat of the chair suggested that the hat had been suspended in this manner for the purpose of examination.
5. The conclusion that Holmes draws from the evidence of the hat about the hat's owner: "He had foresight, but has less now than formerly, pointing to a moral regression, which, when taken with the decline of his fortunes, seems to indicate some evil influence, probably drink, at work upon him. This may account also for the obvious fact that his wife has ceased to love him."
5. The way that Holmes gets a witness to produce evidence by betting him he doesn't know the information Holmes needs: "When you see a man with whiskers of that cut and the 'Pink 'un' protruding out of his pocket, you can always draw him by a bet."
7. The cheerfulness of this story, even in the midst of crimes like theft and assault. Sherlock Holmes seems to be in such a good mood during the whole "Adventure of the Blue Carbuncle." Maybe it's the Christmas spirit.