Yeah, the point of my comparison wasn't to do a point-by-point conversion of The Lord of the Rings to Dungeons & Dragons. It was to point out that even in that work, some items exist that are just more special than others.
In most fantasy fiction, these categories can be hard to sport. However, both artifacts and signature non-artifact items show up routinely. Generally, artifacts drive the story, whereas rare items are those still special items unique to the main hero.
Uncommon is the hard category, as sometimes it's hard to tell when something is "uncommon" rather than "common," or "rare." To steal again from Tolkien, Sting and Glamdring (& Orcrist too) would probably count as uncommon items, possessing special powers to detect orcs/goblins. They're more unique than the simple "magic swords" in the story, like the blades the hobbits find in the Barrow Downs (pretty clearly common). But there are 3 of them, with nearly the same properties, so they're hardly "Rare."
As should be obvious, high-level common (or uncommon) items tend to be "rare" in low-level campaigns. On further reflection, I think I'd put the mithril shirt in this category. While rare, t's not a "Rare" item, it's just a near Epic-level "Common" item in a high heroic (or low Paragon) campaign.
As for Smaug? Unless Black Razor is an arrow of slaying, it's gotta be a combination of a critical hit, and lots of stacked up damage.
Granted, a 21st-level ranger can do like...100+ damage on a crit. But that's a drop in the bucket to an Elder Red Dragon. Granted, it might just have been the finishing shot. But yeah, the implication is that Black Razor is a slaying arrow.
