The scenes that keep the aliens offscreen except for brief glimpses—such as in the cornfield, home video footage, and hell, even trapped in that guy's room—are both startling and quite unsettling, as are the character's reactions.
The pantry scene. Oh, gosh, the pantry scene.
Another example of brief glimpses being scary as hell: a certain scene has an alien arm sneaking behind the kid — just before the lights go off.
The family has just finished getting the house secured when they suddenly realize that they forgot to bring their dog inside. Right at that moment, they hear the dog start barking hysterically, and a moment later she whines in agony and then all is silent until footsteps are suddenly heard on the house.
Just about any scene at all from the moment the aliens enter the house. Why? Because you can hear their clicking, see their shadows, hear their footsteps creak on the rafters, but you never see them. And after watching the rest of the movie, it's almost impossible not to be gripping the arms of your chair, just waiting for something to jump out at any second.
Anything in the dark, or anything even remotely associated with the aliens.
Very early in the movie, Graham is talking to his daughter after she wakes him up complaining about a monster. Graham at one point glances out the window, and he just stares at something for a few seconds. Then it cuts to a shot from his point of view, and we see the silhouette of one of the aliens standing on the roof of his barn.
Before that part, when you hear the cricket chirping. Suddenly it stops.
Some might never be able to look at cornfields, at least right behind your house, the same way again. The taller, the wider, the scarier.
One of the reasons the eventual appearance of the aliens is disappointing is because so much of the film does a marvelous job of building low-grade Paranoia Fuel. Even the film's weaknesses - stilted dialogue, wooden acting, awkward camera work - might have been converted to strengths if the menace of the story had come from the subtle warping of the protagonists' own environment, so that they would question whether the world had gone mad... or they themselves had.