While I do not recognize Muhammad to have been a genuine prophet, I do acknowledge he made some social reforms in Arabia. Of course, one neededn't be a prophet at all to make social reforms.
The reforms he did make:
He required almsgiving, called <I>zakat</I>, in order to create a more equitable sociaety (at his time, his own tribe, the Quraysh, was the wealthiest tribe in Arabia).
He did away with the old practice of burying female babies alive.
He permitted women to own property of their own. Women also became allowed to inherit property.
While he permitted polygamy among the Muslims, it was only if the male could treat all of his wives equally and not favor one over the other.