The events in egypt right now raise a big question: What is a constitution ?
Basically, the legal basis for subjecting people to laws need to be based on consent. A person is subjected to the law of the land because he actually consents to be subjected to it. In theory at least, this must mean that a constitution can only be adopted unanimously.
It is not enough that a majority of the people agree, otherwise, no country will ever grant any minority rights, and no individual rights that go against the view of the majority.
In the begings of early democracies, one could argue that people cannot live in ways that are rejected by a majority people of the country. That argument could hold when people have the right to move away to a more accepting community, or form a community of their own. Especially when there was plenty of legally unclaimed territory. But even then, constitutions like that of the United States of America required adoption by super-majorities, to ensure that at least, sizeable minority were not out of the equation.
Nowadays, people cannot move out of the country, as they are not allowed to by immigration laws of the destination countries. Therefore, there is no legal escape from having uninamity before a could set the rules for the legal and political systems.
Unanimity is practicly impossible to achieve in any universal vote. That is why most new democracies universally elect constituting bodies to draft the constitutions. Those bodies can then deliberate and negociate until they can reach unanimity on a text (unanimity in this context usuall means they believe that is the best they can get, not that they all agree on everything).
This particularly important after revolutions, as there is no basis for legitmity to speak on behalf of everyone. In reforming monarchies particularly, there is historical legitimacy for the monarchy to speak on behalf of the people (without going into much detail, people in a monarchy, at least in theory, have accepted the monarchy to represent them unanimously. Those that didn't accept could hve long ago left the community or the country, or revolted).
Because those representatives unanimously adopt the draft, it is ok to submit it to a universal vote that needs only some form of majority or super-majority to pass.
Now, in Egypt, there is the dangerous situation where those that drafted the constitution only represent a portion of the society, albeit a majority. That is not enough, not by a long shot, for having the right to draft the constitution that will define the rights of the individuals and of the minorities.
Putting such a constitution to a popular vote, and letting it pass if it gets a majority, lacks the legitimacy for subjecting individuals and minorities to its content. Those that will not have been represented in drafting the constitution have every right to reject it, and will only be left with one option : civil disobedience...