It can't be a certain level of performance. First because there is no one performance standard by which to make the cut. But also because even if we did have a solid agreed standard we'd still have to argue over where to draw the line.
It can't be price. We all know really high end gear that is very affordable, including a lot that is better than much more expensive gear.
But it can't be design, or technology, or parts or materials either.
But especially, it can't be mass-marketed and based on a price point. Oh, it can be mass-marketed. And even to a price point. But only if somewhere in there it is apparent that sound quality is a driving factor.
In other words, what atmasphere said: intent.
It sounds silly but its true. Intending to make it high end is what makes it high end.