How much is he involved in the day to day development of their products? I don't know for sure, but being that I am an EE and have worked in an R&D environment, I'd have to say as CEO his input on the engineering level isn't much. Jobs' input is likely more of a "yeah, that's cool, let's develop it further" when his engineers bring him a new doodad, rather than an input like "add two transistors here and we'll go with it".
My guess is there are hundreds, if not thousands of individuals at Apple who share Jobs' "vision". I think the environment they work in is easily as responsible for the products they have produced as any input Jobs has had to those products. In fact, if Jobs can be credited with anything specific that helped Apple gain the edge it has it would be for creating a working environment where imagination of individuals with good ideas can thrive.
The problem is that no one has been as good as Jobs is at promoting the "vision" of Apple. IIRC, when he left the first time, Apple floundered, and it wasn't until he came back that Apple began to improve.
To me, the difference between Apple/Jobs and Microsoft/Gates is that Gates sold computers as a commodity, but Jobs sells desire. So when Gates left, everything kept going, because there are many who can sell commodities, but when Jobs leaves, you need a new leader who can sell desire. That is a bigger set of shoes to fill.
I'm an EE, too, and I understand how product development usually works. The story I have heard is that Jobs will test the prototypes of Apple's products himself before they are released, and tells the developers what's broken or what he wants it to do. This is very much un-CEO-like. If the software crashes, he rips the developers a new one, is what I have heard.
I think that if Jobs is no longer involved in Apple, the stock will swoon for maybe a few weeks to a quarter, after which investors will realize how inexpensive this world class, powerhouse growth stock is now. Without this overhang of “bad news”, AAPL shares would be trading $6-700, easy.