@faraday I think you're almost safe with an either/or here -- system or setting as one people are familiar with. Or, at least, those seem like a far minimized risk. I'm sure KD got more people than they might have simply because they used nWoD as the base system, they just scrapped the world. Similarly, you could have a familiar world and a new system and be about on par there.
I had enough references to stuff people are (or could easily become) familiar with that I wasn't too worried on people grasping the vibe of the setting I had been working on, on the most basic level. Everyone's probably seen at least one Pirates of the Caribbean movie, or some bit of Black Sails or similar shows or movies. 'Pirates, 1715, just if all those creepy legends about sea monsters and ghost ships and all the rest were real'. Not too hard to grasp the basic concept, which felt safe enough on that front. On the era front, it would have had a basic history primer section for folks who wanted to work recent historical events (or be from a place and know what the deal was with said place) with links for further reading if people really wanted to dig in to that aspect more without clogging the wiki with it, and so on, with notations re: anything that diverted from actual history so it wouldn't trip folks up. (This is in addition to 'here are all the cool things about this imaginary setting we made up for you to knock around in' specific setting files.)
@Thenomain I am still a big fan of the 'mission statement' concept on the front page.
Paragraph about intended game tone (sandbox level, metaplot or not, PvE or PvP, competitive or collaborative), paragraph about setting, paragraph about game system. That's 'this is what the main page should be' to me, at least, and it should provide the basic information that would let a potential player have a fairly good idea, at a glance, of whether or not that game is likely to be fun for them or not, based on what they do or don't like in a game.
This is what I'm generally talking about re: 'at least skim something to get a basic idea' -- but you're absolutely right that people need to present this right up front, square one, and not enough people do. (I still think Reno1 did a fantastic job of this, and @tragedyjones deserves props for that.)
Below that, a list of links to basic quickstart materials, walk-throughs, and essentials like policy, setting, and game system info overviews for folks who want to look further before making a final decision.
This crosses over with the 'data organization' thing, but each of those sections, IMO, deserves a 'summary' page, organizing the links to further information in one tidy spot as a quick overview, with links to the further details on these things from there, rather than piling everything into a page and expecting people to slog through it all in one go.