I'm not sure if it's the biggest, but I think it's one that is, in so many ways, at the root/core of so many of "all the other issues" that are out there...
To me, it's the fact that, for a variety of reasons, childhood consists of elements that are either too soon, or just too wrong, for the different ages/stages of development. I'm not thinking, or suggesting, that our culture needs to go back to how things were in the fifties. Things have changed and become more complicated since then. I think, though, the problem is that understanding of what is healthy and wholesome for a child of any age, and understanding the consequences of allowing everyone/everything else to define childhood and then to parent/teach by the "it's how it is anyway" school of thought, or the "they're going to do it anyway" school of thought.
It's very tricky for any parent of any child or a few children to figure out some "perfect balance" of encouraging a child to grow up, allowing him to grow up, and introducing him to just the right exposure to/information about his ever expanding world while also figuring out what is too much for a child of each age.
There's the either/or thinking that's a sign of someone who hasn't had someone else explain that either/or thinking tends to be for the immature. Example: Either you let your ten-year-old hang out with a bunch of thirteen-year-olds OR you're "trying to make a baby out of your ten-year-old". It's not either/or. It''s a matter of thinking out, sorting out, limiting, etc. exposure (in degrees, amount of time, and different settings).
In a more complicated (and in many ways, improved/advanced) society the challenges in doing that thinking out/sorting out/limiting are trickier than ever.
So many (most, regardless of situation) are so much "smarter" so much sooner than so many people realize; while, at the same time, too many people overestimate children's emotional maturity and general maturity while underestimating some aspects of their emotional maturity and/or their intelligence
Without getting into exactly which things seemed to be "fanned" at which time (since, say, 1975/1980). and without a lot of detail here, over the last twenty/thirty years many bars (some seemingly positive, some neutral, some destructive) have been lowered to the point where our society very much and often has the appearance of "better"/"more" childhood; while in reality, childhood has been cut down to be almost non-existent after, say, age four.