IME, it's mostly the statistical information most folks gravitate towards, initially. Everyone wants the Dragonmarks and maybe the idea of belonging to a House/faction, but no one actually cares which house is the oldest or what the internal sub-factions are. The halfling wizard wants to cast cure wounds to back up the cleric, but doesn't care about the fluff around why Jorasco is so adamant that you must pay/charge for the service. Players think it's cool to be able to play pseudo-lycanthropes (shifters) and/or Paladins that favor long bows, but don't want to deal with the fluff around the distrust between the two groups.
You generally get lucky with a few things. The whole table embraces that the Valenar warrior is scary, potentially untrustworthy, and driven by glory even if they don't particularly care about the ancestor worship or other historic aspects. The Phiarlan agent plays up his role as an artist who just happens to be resourceful -- and never actually admits to being a member of the house, let alone Dragonmarked.
I mean, I"ve had those players, but at every table with them I've also had players who, given the opportunity, read most of the player's guide before making a character, and that character has bonds and allies and enemies and strong opinions about who should have won the Last War, and who probably caused The Mourning, and whether Breland still needs a King, and whether the Houses have too much power, and...etc.
Or, they make a Tairnadal elf who literally couldn't care less about any of that, because her whole purpose in life is to do right by her namesake, and her people, and she's too busy trying desperately to not give in to her inexplicable need to teach her fingers to dance the strings of a lute. It's impossible! Her ancestor never played a damn lute! She didn't even like musicians!
ANd meanwhile, and these are my favorite kind of players, another player has made her twin brother, a warlock whose namesake is literally in his head, whispering secrets to him. At first, he was ecstatic, but recently he has begun to fear that his namesake has gone mad, or been corrupted somehow, and he feels that he is slowly losing control of himself, losing himself to the long dead elf for which he was named. He can't ask the elders for help! What if they call him Traitor, ungrateful cur! Surely, he should be honored for his revered ancestor to choose him in this way, right? Right!?
But sure, you also get those players who just want to make Jack the Halfling thief, and never really even remember much about recurring NPCs.
It's jst not my experience that they outnumber the other kids.
I agree that you can't predict the players, though. Which is a big part of why I like getting player buy in when creating a setting. I ask questions about what kind of characters they have been wanting to play, give them an overview of the setting/campaign, and they ask me questions, make suggestions, ask "What if my character comes from a place like [description], and that "q&a" process helps build the world or campaign.
I generally don't make big changes to the world to accommodate a character concept, unless it's
purely or mostly additive (can there be lizard people? Sure, I can find a place for that),
doesn't contradict the setting (is faith rewarded with supernatural ability/is there magic? No, it's a science fiction based setting, but there is some psionic stuff, and we can work out some manner of benefit your character has because of their faith*)
and isn't a huge amount of work to fit in.
If we had fewer campaigns, or if we tended to play really long campaigns, I'd be more willing to make big changes, but the desire to play something like a 4e avenger can wait till we play the next campaign, which won't be a no-magic, low-psionics Alternaty game. Obviously, if the group wasn't interested in such a game, we wouldn't even get that far, we'd be playing something else entirely.
*that one was actually my character, but still a good example of how our group works. Ended up, he had some mental defenses stuff and clarity of mind type stuff that was a result of his faith/meditative practices. He could basically do the Sherlock Holmes deduction thing, and effectively see what had happened in a place, or to a person, etc.