Predictably, many of the answers involved politicized answers like "food and water". But the answer that got voted up the most, by far, was "nurturing parents".
At first I thought, "Good. I'm that. My kids are fine."
Then the question sat with me a little longer, and I started to wonder: what is it to be a nurturing parent? I am engaged with my son enough to know him intimately — what books to buy him (Dance Class #3: African Folk Dance Fever), and what clothes he'd want to wear (he loves the band One Direction.) But predictably, most people do not connect consumerism with nurturing.
Wikipedia says a nurturing parent is one that provides protection for a kid to go off on his or her own and explore. This seems like unschooling. So of course I didn't bother looking for more definitions of a nurturing parent.
Then I realized it's time with the parents. Surely not every parent who spends a lot of time with their kid is nurturing. But you need to spend a lot of time with your kid to be nurturing. Which begs the question, "What should you be doing with that time?" And I think the Wikipedia definition answers that: parents should give the kids freedom to explore.
So I'm going to say that it used to be that people thought the unfair advantage was top-tier schooling. That's why we have a huge controversy in the courts about affirmative action, and we have schools like Harvard guaranteeing everyone enough money to go there if they get in.
But at this point in our society's evolution, education is not an unfair advantage so much as a waste of time. And the unfair advantage is being born to parents who know that.