Western society, to speak generally and, perhaps a bit optimistically, is "liberal" enough to allow for most types of romantic unions to be recognized as legitmate. Gender is no longer the primary definition of legitimate unions. Love is.
For this reason, it seems that a complete relaxation of marriage law would have a limited effect on actual unions.
I agree with post 3 that there has to be some balance between rights and protection. For consenting adults, there really shouldn't be restrictions on marriage. Some places have more restrictions than others. Some restrictions are no longer necessary. There's not as much danger of an entire population marrying their cousins and in-laws these days. As far as the age restrictions go, I think some of them are necessary. Children should be protected. They are not able to consent to the type of relationship implied in a marriage.
I think that for same sex couples, the difference would be very significant. Marriage may not seem that important, but it is not really about marriage. It is about rights. A society where same sex couples could marry would also be one where they had equal rights, and that would make a big difference. Allowing them to marry would not be the significant part. A society where they are equal would be significant.
Aside from offending those who have religious convictions regarding marriage, I doubt if easing all marriage limitations would really change society very much. I guess it might complicate matters such as health care if somebody had multiple wives and a whole bunch of kids (they'd need to add a tier to "family coverage).
I agree 100% with post number 2, at least when it comes to gender. With respect to age, however, I think that such freedoms could lead to unions that were not necessarily consensual, as children are not really prepared to make such decisions or to live as equal partners in a marriage. Similarly, I would argue that marriage ought to be defined as a union between two people, as it has to be an equal partnership. So total freedom of marriage (which I've not heard anyone argue for in our politics today) might not be acceptable to most people, as it could lead to exploitative relationships enshrined by law.
In a sense, I think that it would not really matter. The vast majority of people would get married in one man-one woman marriages and only after they were "of age." There would be some same-sex marriages, of course, but I don't see those really changing our society since there are already same-sex couples who just aren't married. As for anything else, the numbers would likely be very small and not really likely to change society.