The movie was like an origin story for Disney's first Super Hero Family (...not the Fantastic 4)
No one really expected a sequel, (even if we wanted one) until it was announced that Cars was getting a sequel first... Cars was arguably their weakest movie up to that point (which is now Cars 2). And then Monsters Inc and Finding Nemo (findiing Nemo!?) were getting sequels.... OUTRAGE!!
Of all the movies that Pixar has made, outside of Toy Story, The Incredibles should have been the first one to get a sequel, or an animated Saturday morning cartoon, or some form of follow-up.
And Brad Bird is coming back to Pixar to make it happen too.