Let me answer it. What will be lost is the same thing that would be lost if they had them wear bright yellow and green costumes, if Ben was gay, if Sue was fat, if Reed was played by a 5 foot 2 inch tall actor. Does it necessarily change their character? No. But at what point do any of these changes cause this to no longer be the Fantastic Four? Do any one of these changes make it so? To some people, any single one of those changes would.
Bottom line is there are people on here to whom seeing as literal a translation of the FF to the screen as is possible within the context of being sensible (I'm sure we don't literally want Stan's dated dialogue in the movie!) is the most important thing. To others, their own social agenda is more important than the literal translation. It comes down to a personal preference. One is not more right or wrong than another.
Even the answer to the question of 'at what point do these changes cause this to cease being the FF?' is a personal matter. For some, changing the race of any of the characters wouldn't cause it NOT to be the FF but changing their uniforms drastically would. Or making Ben gay would not but having Sue played by an overweight actress would. All of these changes could actually be done and not change the core of who the characters are but as has been pointed out, comics are a visual medium and most on here who have been comic fans for a long enough time probably looked at the pictures before they could even read very well or understand some of the more complex elements going on in the stories they were looking at pictures of. Changing a character from black to white or vice versa is as big a visual change as having Reed be short and fat or having them all wear day-glow orange unis.
The choice is obviously up to the director and the studio. The response will no doubt be viewed as determining if it was the right choice or not. So let the decision be made and let the chips fall where they may.