gonzo: Over the years one of my considerations in owning a vehicle was how easy it is to change spark plugs. One thing they have improved on that you did not mention is on newer vehicles you see "Iridium" type plugs being used, good for over 100,000 miles, at least that's the claim! So you don't have to do it as much. If your in the business of reparing cars, I don't know if that's a good thing, but for the automobile owner I guess it is.
Also, what about the vehicles that used "wasted spark" This meant you had to change two spark plugs per cylinder. Gonzo, I bet you could come up with stories just on this one , trying to explain this to customers why a 4 cylinder needs 8 spark plugs replaced!