When populations migrated elsewhere on Earth, they always had the same air to breathe and the same gravity to live on.

If humans are born under a very different gravitation than earth's (lower) then it's going to be an ordeal for them to be able to visit Earth.
They won't be free (as they'll have to wear an equipment all the time) and this will distinguish them from the other earthlings.
And how can they have their point if only a few of them exist?
The other problem is that there's no open world out there; everywhere humans go will be in an enclosed environment (with very tough discipline - for safety reasons) and that is not a good way for humans to develope (grow).
And, compared to 4 centuries ago, now there are machines, robots and birth in the future will be more and more controled.

The comparison (about Mars) americans make with the settlers coming from Europe and migrating to the New World is not seen at all the same way, over here. Precisely because so much has changed and there are no worlds similar enough to Earth in the solar system to make migration
envisageable. Meanwhile, robots are evolving so fast while nobody has found a way to shrink human beings.
I'm convinced the Mars Society's message is a cultural trend that just like O'Neill's space colonies will last a while, a short while.

Raoul