Instead of becoming a big rock lump, Ben Grimm is granted the power of John Carpenter's Thing by those cosmic rays. He can sprout tentacles and jaws and all that gross shapeshifting, and he can assimilate people, effectively cloning his mind while creating new bodies for himself in the image of those he assimilates. As with canon Thing, Reed is incapable of curing him. Ben's personality is completely unchanged by gaining these powers.
How does the FF's history change with this version of The Thing?