The massive restriction that the central characters face as clones is that they are not free to live their lives as they would like and that they do not have ultimate control over their futures. This is the shaping reality that becomes more and more clear as we read this excellent book. It starts from the beginning, as we hear the kind of restrictions that the children face at Hailsham as they are told not to smoke or do anything that could harm their bodies, because of their "special" status, and then of course we see this as they move towards adulthood and Tommy and Kathy form a relationship based on true love that is doomed to a tragic ending.
This novel above all paints a very bleak and unyielding picture of what it is to have life but with no control over it. The moments of happiness that Kathy and Tommy enjoy in their relationship are only brief glimpses of joy against an unyielding background of gloom and depression as we see the kind of destiny all the clones must face. The clones are ultimately presented as humans, but humans who are unable to actually live fully as humans because of the way that they are used for harvesting. This is the massive restriction that exists in this world.