What was strange about the reaction to the airplane?
The airplane fly-over is one of the first events that lead us, as readers, to realize that something is very wrong in Jonas's world. His world isn't just different or strange; there's something just not right about it.
The intensity of the reaction alone should give you a few clues about its strangeness. Think about it: when was the last time you even noticed a plane flying over? Compare your reaction to planes with those of The Giver's characters, and you'll see what was so strange.