I figure that might just be it.. notably it uses the windows install key as a "unique identifier". For macs and such who knows, there's probably something similar. Valve might say somewhere, but offhand I'm skeptical they'd say something which is key to their encryption and copyright DRM system... it'd be a quick way to narrow down how to break it.
Of course, people HAVE broken it, sort of, as seen by the fact that pirates keep coming about here, then going "how do I install this mod, I don't have the steam version" and promptly getting tossed back into the deeps of the internet. Still, that's no reason for Valve to go and let the specifics be free to every script kiddie out there.
Really, while you can do a full copy of the steam directory, most of it isn't nescessary and the stuff would be added with an install and log-in to your account. Heck, if you go the steam cloud route for savegames, all you would want is to copy the steamapps directory, as it's the only thing of note. Steam is intentionally computer-unspecific (unlike stupid securom, which if you have a game sold over steam that ALSO uses it, like say the Batman games, will cause issues and probably force you to re-download it or parts of it), so you can access your profile from any machine you log in to. Of course only one machine at a time, but that's their DRM system.
Good luck with your new computer and all!