Just it's looks. It was a much better car than the Vega/Pinto and the shape was aerodynamic. Modern SUV's are basically super sized gremlins, and the Prius isn't far off.
They have shorter hoods, but pedestrian safety regs are changing all of that. The new Honda CR-Z is basically a Honda Gremlin.
There was nothing wrong with the Gremlin, like someone who's just generally fugly even though they have no specifically awful features. The big windshield meant it was always 200 degrees inside when you got into it during the day, and after a year the seats and dash were all sun bleached.