What makes Steampunk, Steampunk?

[MetalRenard]

Hello!

I've recently received an e-mail from someone asking if I could make some Steampunk rock music and the request got my attention.
Steampunk... And rock... Well I can do the rock side of things, but what would make a piece "Steampunk"? Yes, I could add mechanical squeals, battered rusted metal and the occasional engine sound, but what else am I missing?

There must be more to steam-punk than that, and so I put the question out to you all - what would make a piece of music unavoidably "Steampunk"? :)

Thanks!