Artistic License - Biology: The four-limbed ants, the many male ants, the scaling of most characters, and many colours. Also ants fearing any fall at all. It was mostly deliberate with the designing of the characters, and maintaining the Rule of Funny.
But Hopper strangling Flik at the climax - ants don't have lungs.
Grasshoppers can't actually fly like that. True, they can use their wings to propel themselves short distances (hence the name), but they don't actually fly. They're being confused with migratory locusts.
Shouldn't the queen be sitting underground, swollen with eggs and unable to move?
This one is actually justifiable: some species of ant have queens that are perfectly mobile and no bigger than the workers, even when laying.
A great deal of the insects presented would also be more likely to eat each other, like Manny to his wife, or Francis to Aphie.
Executive Meddling: For the confrontation in the bar, Pixar's animators had planned out a large, anime-esque sequence, not unlike the one later seen in the movie Horton Hears a Who!. Disney suits didn't understand the scene as it was laid out to them, and the gag was reduced to Francis saying, "Shoo, fly, don't bother me."
Remember that mosquito who ordered a pint of blood and drank himself bloated? That's Squidward.
Science Marches On: Dim was supposed to be a fictional variety of generic rhinoceros beetle. Then a couple years ago they found a rhino beetle that sort of looks like him.
Watch when the ants Zerg Rush the grasshoppers. The grasshoppers all run for it - except for Molt, Hopper's brother, who actually dives into the grass in terror. So he doesn't abandon his brother, and that's why he's still around at the end.
He also sheds his entire chitin skin all at once as he makes the dive.