This article is an able try: http://unleadedwriting.com/2014/07/10/what-does-steampunk-mean-10-authors-share-their-vision-of-the-genre/?utm_content=buffer810fe&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer
I do not have the time to make my go at it in full here. But there is one phrase that guides me when I write it: “The Future That Never Was.” Even though my Steam is on another world, it’s very much informed by the dreams and aspirations of the Victorian Era. Not that there aren’t problems, and plenty to subvert. But to me, Steampunk, well done, is an OPTIMISTIC genre. Regardless of the politics of the writer, there ought to be hope, vision, and a sense of the ability to make things better.
It’s also a genre driven, if I may say, by the Rule of Cool: http://tvtropes.org/pmwiki/pmwiki.php/Main/RuleOfCool That is to say: “The limit of the Willing Suspension of Disbelief for a given element is directly proportional to the element’s awesomeness.” Airship dreadnoughts flown by mechanical automatons! Absolutely! Airships that make everything in the world black, disgusting, and sick? Well…you just icked the wow of steampunk. Even China Mieville knew how to skirt this line. Bas-Lug could be fairly depressing. But the monsters and clockwork machines were just that wicked.
At the end of the day, there’s another phrase from Brass Goggles that I loved: “Steampunk is as a Steampunk does.” At its heart, it’s an attitude. And not a dour one.