Apropos the iPad, I came on Jeff Raskin’s 1979 “Design Considerations for an Anthropophilic Computer” in the Stanford archives. Here’s an excerpt:
This is an outline for a computer designed for the Person In The Street (or, to abbreviate: the PITS); one that will be truly pleasant to use, that will require the user to do nothing that will threaten his or her perverse delight in being able to say: "I don't know the first thing about computers," and one which will be profitable to sell, service and provide software for.
You might think that any number of computers have been designed with these criteria in mind, but not so. Any system which requires a user to ever see the interior, for any reason, does not meet these specifications. There must not be additional ROMS, RAMS, boards or accessories except those that can be understood by the PITS as a separate appliance. For example, an auxiliary printer can be sold, but a parallel interface cannot. As a rule of thumb, if an item does not stand on a table by itself, and if it does not have its own case, or if it does not look like a complete consumer item in [and] of itself, then it is taboo.
If the computer must be opened for any reason other than repair (for which our prospective user must be assumed incompetent) even at the dealer's, then it does not meet our requirements.
Seeing the guts is taboo. Things in sockets is taboo (unless to make servicing cheaper without imposing too large an initial cost). Billions of keys on the keyboard is taboo. Computerese is taboo. Large manuals, or many of them (large manuals are a sure sign of bad design) is taboo. Self- instructional programs are NOT taboo.
There must not be a plethora of configurations. It is better to offer a variety of case colors than to have variable amounts of memory. It is better to manufacture versions in Early American, Contemporary, and Louis XIV than to have any external wires beyond a power cord.
And you get ten points if you can eliminate the power cord.
Any differences between models that do not have to be documented in a user's manual are OK. Any other differences are not.