For a sphere, E falls off with 1/r^2, for an infinite line charge, E falls off with 1/r; for an infinite sheet of charge it's independent of r. However, according to Coulomb's Law, the electric fields fall off as 1/r^2. So why don't the electric fields for the infinite line and infinite sheet of charge also fall off in the same way?
Hint: As the Gaussian surface gets very far from the source charge, look at what its characteristics depend on.