Some observations about steampunk tropes after spending too long with brass and gaslight:
The mad inventor is steampunk's most reliable character, and probably its most misused one. In most manuscripts, the inventor exists to explain the technology. In the books people actually remember, the inventor exists because they can't stop building things even when the building is what's ruining their life.
Airships work best when they're treated like ships, not like magic carpets. The good ones have crews with hierarchies, fuel that runs out, weather that matters. The moment an airship can go anywhere without cost, it becomes furniture.
Cherie Priest said something in an interview that stuck with me: "I tend to write about people who struggle to define themselves." That's the through-line of Boneshaker, a book often remembered for its zombie gas and walled-off Seattle. But the actual engine of the story is a mother walking into a poisoned city to find her son, because she's spent years being defined by her husband's catastrophe and this is the first thing she's done that's entirely hers. Most steampunk centers an inventor or an adventurer. Priest centered a parent making an irrational choice out of love, and the genre bent around her.
Clockwork automatons are one of those tropes that reveal what a writer really thinks about consciousness. If your automaton is just a robot with gears, fine. But the interesting versions are the ones where the writer isn't sure whether the machine is aware, and lets that uncertainty sit on the page without resolving it.
The goggles-and-corsets version of steampunk might be the genre's biggest obstacle right now. China Miéville put it bluntly in an interview with Lightspeed Magazine: a lot of steampunk has become focused on "the endless replication of corsets and zeppelins and strange steampunky eyeglasses," which he called "a completely deracinated aesthetic." He's right that the visual shorthand can become a substitute for the actual work. When the costume is the genre, you're writing setting, not story.
Class conflict is the trope that separates steampunk from standard fantasy. Fantasy has kingdoms and peasants. Steampunk has factory owners and the people whose lungs the factory is destroying. The specificity of industrial-era class dynamics gives steampunk writers a kind of built-in tension that sword-and-sorcery has to manufacture from scratch.
Secret societies in steampunk almost always work better when they're protecting something mundane. A secret society guarding ancient magic is a fantasy trope in a top hat. A secret society controlling the patent system, or suppressing a technology that would collapse the coal economy, that's a steampunk trope with teeth.
Philip Reeve's Mortal Engines is famous for its premise of mobile predator cities that consume smaller towns. But the thing that actually makes the series work is how absurd and logical the concept is at the same time. Municipal Darwinism sounds ridiculous until you realize it's just colonialism with wheels, and then it sounds exactly right. Reeve went further than the usual magnificent-but-broken technology angle: the entire civilization is a machine that's breaking, and most of the characters are too busy surviving inside it to see the whole picture.