These attributes could be character archetypes or traits, attributes of the setting, characteristics of plot or pacing, and broader themes. Essentially, I'd like to build a definition of the genre by identifying its basic components.
Please provide examples to back up your claims.
Generally speaking, Steampunk concerns itself with Victorian characters, themes, and technology level, while taking the technology available and creating devices that, if not impossible, are impractical. The spider vehicle in the film Wild Wild West.
For technology, Steampunk typically cannot use internal combustion engines, nor most forms of "regular" electricity. It is still perfectly acceptable to make use of capacitors or tesla coils, or basically anything found in Dr. Frankenstein's lab. Steam engines, hydraulics, and clockwork are the ways in which to make things. And the more impractical the better. Best example is the device that is the size of a barn that makes ice cubes in Back to the Future Part 3.
Regular propeller airplane? Nope! Must use dirigibles, or bizarre flying contraptions out of Leonardo Da Vinci's sketchbook. Jets are completely out. See the 1961 film Master of the World.
For any device created, it is highly important to have it finely crafted and visually appealing as well as functional. For example, if one were building a rocketship to the moon, it is not simply a matter of putting dials and guages (although there should be as many of these as possible), but there should also be a rather posh and luxurious chair in which to sit while operating the levers and dials on the overly crowded control panel. From the Earth to the Moon by Jules Verne.
Main sources include anything by Jules Verne, a great many things by Edgar Rice Burroughs, and the television series Wild Wild West.
Women and Men have Victorian clothes and Victorian traits. These include romanticism, a high regard for virtue, chivalry, and outright chauvinism. At least for the good guys. The bad guys are really bad and over the top, and usually with some sort of very fancy mustache. The villians of this genre are much like James Bond villains.
Steampunk is a romantic sub genre of science fiction/fantasy. The types of device typically found in Steampunk are enumerated in Ernest's answer as are the outward behaviours of steampunk characters within their universe.
As to the why of it all. The modern age is one where visions of utopian empire building are frowned upon and ridiculed. We are so close to an actual ramshackle global government no one asked for that the thought of one in modern clothing is abhorrent to most.
The people of the Western world in the late 19th Century had no such embarassment. The 19th Century was the last time that there were real frontiers, where empires were seen as "a good thing" and where man was confident that soon we would have worked out the mechanics of the universe itself.
People of this era saw nothing wrong with imposing their civilisation on others, they believed naively but with great fervour in the idea of a brave new world. Optimism was high.
Steampunk as a genre attempts to reconnect us with the idea of utopia and also allows us to satirise it almost without irony even as we embrace it. The idea of fantastical devices all built in ecologically unsound materials and manned by a people filled with bravura, boundless confidence and hope for a brighter tomorrow allows us indulgent fantasy, wry humour and an amazing design ethic in one package.
The fact is steam punk art looks gorgeous, the narrative side of the genre struggles to keep up with the imagery. "Proper" steampunk glories in the visual while at the same time acknowledging that utopia is trickier than the characters may believe it to be. It's very hard to depict such naive and often politically imprudent characters while preserving some respect and admiration for them. The best people at this have been, ironically, the Japanese. Much anime steampunk abandons the notion that utopia is a naive hope, I think the Japanese have had a very utopian worldview in the past few decades. The anime "Metropolis" presents the most coherent steampunk world view I have seen to date. Steamboy looks great but isn't quite as accessible. Howl's Moving Castle mixes steampunk with straight fantasy and suffers a little for it.
Essentially if writing steampunk the authorial voice should be indulgent, expansive, epic but always tongue in cheek.
