The early Celts believed in "thin places": geographical locations scattered throughout Ireland where a person experiences only a very thin divide between past, present, and future times; places where a person is somehow able, possibly only for a moment, to encounter a more ancient reality within present time; or places where perhaps only in a glance we are somehow transported into the future.
After September 11, St. Paul's Chapel functioned for many as a "thin space" of refuge and hope--as memorialized in Peter Ostroushko's "Meditation of the Thin Space from St. Paul's Chapel." In the Celtic tradition these spaces where the line between heaven and earth seemed close to vanishing were, as I imagine it, quite distinctly set apart from the spaces of ordinary life.
The point of the megachurch seems to be quite the opposite, the object of the architecture being to make the worshippers feel as if they're in some larger version of their living room, or rather their media center. From the outside they look like a corporate headquarters. The cavernous insides seem more like sports arenas, with the broadcast technology to match.