I don't think I've ever been anywhere like Heathrow Airport, and maybe I never will. Where else in a European country do you find such a large built-up area dedicated to a single enterprise? You enter the sprawl of roads, car parks, service buildings, vents, trains, coach stops, terminals, radar towers, and everything around you is centred on a single activity. I don't think even a nuclear power plant or oil refinery would match the scale - there's not the same system of businesses supporting them or trying to make money from the people who pass through.
As soon as you enter this huge system you're channelled and controlled. It starts well before the security line; the airport's transport system extends arms outwards. When you're on foot, it's not just the worry about being late that keeps you from deviating and wandering around the airport. Maybe the no entrance, no trespassing signs aren't everywhere, but you know you're not meant to stray - it's not what the airport's there for.
Past the security theatre, airports are modernist icons in a postmodern world. Futuristic international modernism has begat postmodern nowhere places. The business of moving people and things around is now distinctly old-fashioned; the wireless internet and LCD-screen ads are a facade over the top of this. (And perhaps they highlight the point that greater telecommunications make people travel more rather than less.)
My favourite airports (as I am a seasoned traveller) are those where the boarding tubes don't seal completely, or you disembark onto the tarmac rather than straight into the terminal - as you move between sterile, air-conditioned worlds it's a little glimpse and breath of the outside, and the place where you are.