The laws of physics themselves may also be drastically different from Prime to Prime and from one plane to another. (I seem to vaguely recall that Oerth is one world where smoke powder literally doesn't work; is this correct?) The local laws of physics may literally not allow for the invention of firearms, much less electronics.
There are also cultural factors that may prevent the invention/widespread use of technologies. For example, even some of the ancient Greeks understood some principles of both automation and steam power! However, many ancient societies, including ancient Greece, had a means other than technology to get stuff done: a hot and cold running supply of slave labor. Why build an expensive contraption to do something when you could buy slaves cheaply at the local market to do the same thing? A typical fantasy society has access to magic, lots and lots of it, to get things done. Someone wanting to do some project would likely just hire a mage or research a new spell rather than trying to build a new magic-less invention.
I believe that some ancient Earth societies also didn't really see the need to mass-produce a new technological marvel or put it into general, widespread use. A new technology was something to put into a temple (to honor the Gods and impress visitors to temples?) or a curiosity, maybe something to impress one's peers, but not something really designed to make people's lives easier or to be used routinely by the 'masses'/lower classes. Large-scale industrial production assumes a certain cultural mindset.
Amazing post, Jem. Can't really say anything else.