Broadly speaking, I think Steampunk is speculative fiction connected to the era dominated by steam technology, and that should include many things: alternate history, science fiction, fantasy, the paranormal, and so on.I like this definition. It's nice and inclusive, rather than trying to make one kind of steampunk "better" or "truer" or "more authentic" or "worthier" than other kinds.
I essentially ended up sliding sideways into the world of steampunk from the world of historical fantasy (Naomi Novik's Temeraire series, Susannah Clarke's books, Sorcery and Cecilia, etc.) so it's distinctly likely that those kinds of books have influenced my preferences where steampunk stories are concerned. I find that the elements which really categorize the idea of steampunk for me include 1) an underlying sense of optimism in the ingenuity of humanity to solve problems even in what may seem a dystopic society 2) good old-fashioned 'adventure' 3) unusual, not to say anachronistic technology 4) some kind of kinship with (despite departure from) Earth's actual history. I like best the sorts of stories which incorporate recognizable historic figures or events and then take an interesting tangent off what we know.
(That said, one of my current projects is a steampunk Sleeping Beauty, so clearly I'm not into restricting the genre either. *wry grin*)
InsertPunk Genres generally are, I think, punk because of the DIY aesthetic. Looking at Cyberpunk for example, it's about what happens when powerful technology falls into the hands of the masses. It's not just what it *is*, it's what you *do* with it.
In the case of Steampunk in particular that's driven it's an idealized "never was" world, where we take elements of the past and filter them through our own cultural perceptions. IE, we are putting our methods of production into the hands of the people of a previous world. (More or less, depending on the author.)
One other thing I'm trying hard to get into my books is what I call "the quest for authenticity". If you look at the maker apsects (costumes and craft projects) you'll see that a great deal of what makes Steampunk resonate for people is the idea of something handcrafted and personal in a world of mass-produced items. It also has an obsession with materials that are far less easier to manipulate and craft than plastic such as brass, leather, and steel.