To start, the enormity of the complex approximates the vast sameness you would expect from floating in a dimensionless limbo. Ground to ceiling, the place soars to thirty feet in height. Corridors stretch in seemingly endless straight lines -- looking far down along their paths, your eyesight gives out before you get even halfway through.
Walking the corridors end-to-end gives this bizarre feeling that you're not really going anywhere. Everything looks the same. It makes you question whether the sensation of the ground under your feet is truly real. Marching down the moving walkways doesn't help much. They just get you to your nowhere destination faster, if you ever get there at all.
And then there's the feeling of hollowness. The place is so big that whole sections are devoid of travelers. And when you do come across some lost souls, they've always got this blank expression that reflects how utterly lost they (and you) are.
Perhaps worst of all, the airport is quiet. I suppose when you're sent to purgatory, you're sent there to repent alone, in silence.