As a writer who has published in the genre, what is steampunk to you?
At least that's my take on it.
Why is it an setting instead of a genre? Well, because much like I say about pulp or super hero stories, you can tell all kinds of genre tales within a setting of steampunk. As long as you follow "the rules," there's nothing stopping you from telling a horror tale, a romantic drama, a melodrama, a piece of historical fiction, an action/adventure story, a science fiction tale, or even a fantasy story. Any genre you want to write is available to you providing you obey the setting.
What does that mean? Let's see. It can be defined several ways and each writer and/or publisher seems to have their own criteria.
1. It can be defined by time period and geographic place. For example, some strict and rigid keepers of the steampunk law deem that the late 1800s and the British world of that time are the "true" criteria for "geniune" steampunk. Some less strict lawgivers are willing to budge a little on the place (allowing the U.S. West or the Eastern world) as long as the time period is adhered to.
2. It can be defined by costumes and fashion. For some, steampunk is about corsets and goggles, and as long as your protagonists and antagonists dress the part, you're on solid ground, even if it means transporting Victorian living to some other time and place, whether past or future.
3. It can be defined by technology. For some, as long as the world about which you write is based in steam-driven (or at least non-electronic-driven machinery) tech, the time, place, and fashion are flexible, even allowing forays into a steam-powered fantasy land (such as a re-imagined Oz or another planet).
Of the three, my personal favorite is to follow the technology and no matter the time and place to have a feel of late 1800s society, whether Eastern, Western, or In-Between-ern.