There's been a really nice discussion going around on Ed Felten's analogy quest for the Almost General Purpose Computer. With the discussion still on-going, his favorite comparison is the Almost General Purpose Language.
Now, Almost General Purpose Computers (AGPCs) don't start off that way. In almost every case, the starting point is a general purpose computer, or at least general purpose components. Either restrictions are added, or the software is deliberately constructed so as to have limited functionality. The key point is intent - AGPCs happen by design! (They can also happen by incompetence, but I can't see any useful results down that line of thought right now.)
From this comes my main objection to most of the analogies to date - although many of them capture the idea of the AGPC, they don't elicit that gut-grabbing reaction that says this is wrong. The technical accuracy of the language comparison succeeds too well, by continuing with a rather dry, academic feel to the questions. For most of us, upon being presented with an Almost General Purpose Computer or Language, the immediate reaction is What are you hiding? or What can't I do?
My preference is to find a comparison from the legal arena that pulls up the same red-flag instincts that technologists get from the idea of the AGPC. Discussion comparing AGPCs to concepts from a 1940's novel just doesn't pull on the gut the same way.
To correct this, we need to identify the group we want to make feel this way. Felten's epiphany comes from Washington, land of lawmakers and lawyers. So we'll pull out a red-flag raising legal concept here.
The Almost-General-Purpose Computer is Prior Restraint.
For a legal definition, the closest dictionary at hand is at law.com; this is start of their definition of prior restraint:
prior restraint n. an attempt to prevent publication or broadcast of any statement, which is an unconstitutional restraint on free speech and free press (even in the guise of an anti-nuisance ordinance)
Of course, not every single action by a computer consists of publishing. But it's not the big stretch you might expect. The origins of copyright as a body of law stem from the time when technology suddenly made multiple copies easy and cheap. Computing technology has pushed "easy" and "cheap" to an extent never imagined by the originators of copyright.
Foregoing prior restraint does not remove the requirement to publish responsibly. If you're crossing the line, you will be held accountable for what you do. But it will be for what you do, not for what you might do.
That's the key element I want captured with regards to general-purpose computers. There may very well be some things you shouldn't do with a computer, things that are illegal, immoral, both, or just a really bad idea. But at the same time, there are unexpected things - things we didn't even know we could do. For all the trouble it is causing the audio recording industry, compressed audio implementations in software constitute an amazing development - one not even predicted by some of the developers of audio ocmpression. Society as a whole has a poweful new communications tool available as a reulst. You don't want to risk losing those things.
And that's why we dont't want either Prior Restraint or Almost General Purpose Computers.