Just what the hell is ‘Steampunk’ anyway?

I realised something the other day. While I’m perfectly happy to point at a book and say, “it has elements of Steampunk in it, it’s a Steampunk novel” I couldn’t actually define Steampunk in a couple of sentences. (And why is it my Mum who always somehow manages to stump me like this? Dammit!)

I know that Steampunk resides in a Victorian setting although it’s not always found in Victorian times (I’m looking at you China Mieville). There’s a higher level of technology than you would expect to find historically but it’s all driven by coal or steam (funnily enough). The emphasis of the plot (in least in what I’ve read) is on high adventure, mostly carried out by bored gentry or ladies with ideas far above their station.
There are goggles in abundance but they’re not just any old goggles, oh no. Steampunk adventurers don’t wear goggles, they all wear devices such as ‘Dr. Sanford’s Optical Enhancement Aid’. Same deal with the weaponry. See that dashing young Lord of the Realm who has just spied the mad scientist about to blow up the dirigible (there is always a dirigible)? The mad scientist isn’t up for negotiation here so our hero has no choice but to employ ‘Professor Barking’s Amplified Lightening Conductor’ (‘ray gun’ to the likes of you and I) to save the day!

Now I just know that there’s a lot more to Steampunk than what I’ve light-heartedly covered; genres and sub-genres always seem to come into being with a lot more thought behind them than someone saying, “wouldn’t it be really cool if the hero wore goggles?” What is Steampunk all about once you put the goggles and dirigibles to one side though? And don’t even get me started on Teslapunk…

Can anyone help me out here? In one of those moments of irony, Vandermeer’s ‘Steampunk II’ is lurking around the house somewhere but I can’t seem to find it…