One of my most successful D&D campaigns of all time was set in: "Uh, D&D."
Pretty much I got blindsided one day by some friends wanting to play D&D, so we just played some D&D. Everything about the setting got made up on the spot. Cities, nations, etc, when they were needed.
It congealed into a decent setting, everybody thought I'd had it up my sleeve the whole time, but it just sort of built organically from the first storyline.
So I don't think players particularly CAN care about stuff they don't know about.
One of the most "huh" games I've ever ran was an Eberron one-shot ... I'd dissiminated via email and a website a pretty comprehensive compact run-down of the Eberron setting and the interactions of the nations and races, etc etc. Got to the table ... nobody had bothered to read any of it, nobody had any idea what was going on "behind the scenes". I had to explain "what you need to know" about the one little area the adventure was in ... I guess everybody had fun, but really, it could have been set in "D&Dland". In fact, had I been prepared to have to tell everybody about everything by pulling it out of my rear, it probably would have been BETTER. I certainly could have saved alot of time putting together the micro-handbook-to-Eberron that nobody looked at and it probably would have had a little more "punch" knowing it was just what -I- was making it for that one game.
--fje