Duergar are silly.
There is the germ of a good idea lurking in the Duergar soul, but they are in essence, concept, and execution silly.
It starts with their name. Duergar (pronounced "DWER-gar") hardly rolls off the tongue; that's not really their fault, though. Although the Duergar mythos seems more akin to the Norse "Dvergar" (dark dwarves), the particular D&D spelling is lifted straight from the Northumberland dwarf-men of English mythology.
The silliness continues in their concept. Dark Elves, their spiritual kin, have a strong hook - they're backwards elves. They live in caves instead of forests, they worship evil instead of good, and they have dark skin instead of fair. Everyone can get on board with the idea of things having an evil twin.
Duergar don't quite make the cut, though. As "backwards dwarves" go, they're sub-par. Their "evilness" largely amounts to living underground, being relentlessly mercenary, and having a fundamentally grumpy disposition. An untrained observer might find it tough distinguishing them from the good version.
It gets worse. Time and subsequent editions have not been kind to the dark dwarves.
D&D 3.5 established that Duergar were naturally psionic. Possibly it was hoped that "evil psychic dwarves" was more memorable than mere "evil dwarves". Among the inherent powers of these psychic dwarves was the ability to increase their size, becoming giant dwarves. (As Order of the Stick points out, an oxymoron.) Evil psychic dwarves have many excellent roles in literature, all of them played by Michael J Anderson, but the ability to "hulk out" defeats any aspirations the psionic Duergar had to pulling off "creepy" rather than "ludicrous".
4th Edition delivers another savage beating to Duergar self-respect. Says Thunderspire Labyrinth: "Duergar beards and hair conceal long, stiff spines that carry a burning venom. They can pluck and hurl these quills like daggers."
Apparently so ubiquitous is this quill-plucking that the Monster Manual 2 defines it as their racial trait. Dragonborn breathe fire, Eladrin teleport and Duergar throw their hair at people. This is in addition to their psionics, apparently - the Duergar Champion is still kitted out with his super-growth hormones, and will not hesitate to deploy a Large miniature base the second he gets bloodied.
These, then, are the theme enemy for the second of Thunderspire Labyrinth's mini-dungeons. As players get ready to explore the Horned Hold, they'll be looking forward to a whole mess of fighting against size-changing mind-reading hair-throwing little people. The stuff epics are made of.