A recent post to some of the Steampunk communities on Livejournal got everyone discussing what the "sub-culture" actually is. Or if it really is a sub-culture to begin with.
I look at the characteristics of the community as governed less by rules or guidelines, and more by various "personal ethics."
Some people want functional steam, and see cogs-as-decoration as an unfavourable affectation, while others favour appearance over function. I read that some people are neo-Victorian re-creators, focused on the finer points of pleats and buttons while others are more like LARPers who seek to inhabit a world that never existed and/or a time that never was. Some people buy off the rack, others have a strict DIY mentality.
It can all be called Steampunk, and it's all awesome.
The only umbrella concept I can see is that steam is the prime source of motive power. (hence the name). This is anachronistic to begin with, but if you start layering on more and more anachronisms, the more punky it seems to become. Most of the other ideas that most think of as "Steampunk" are either corollaries of this, or are preconditions that lead to steam being king.
The original poster likes coming at steam from a post-apocalyptic future where much has been lost. In my opinion, this is still steampunk, from a certain point of view:) Kind of like how the TV shows "Legend" or "Wild Wild West" is US Civil War meets tech, while "Firefly" is high tech meets Civil war. One is retro-futurism, while the other is future-retroism. And you got your peanut-butter in my chocolate.
My personal guiding question for steampunk design and fashion is:
"What if all those steam powered inventions from the Victorian era were not hindered by such mundane concepts like 'practicality' or 'manufacturability', given that electricity was never anything more than a novelty, and it didn't really catch on."