What circumstances promoted and inhibited social reform in the early republic?
I would say the social homogeneity helped to inhibit social reform in the early republic. The United States started off as a fairly singular society. Most of its inhabitants were white settlers who came from Europe and enjoyed, for the most part, a similar economic experience. Most experienced the same religious or spiritual sensations and those who didn't were discarded as "sinners," and automatically dismissed from the discourse. There were wealthy and there were poor individuals, but for the most part, I don't sense that there was such a divergent narrative. Native Americans were either killed off or enslaved, as were Africans brought over for human trafficking and profit. These narratives were not validated by much of the social order. The lack of diversity helped to shrink the moral imagination of the settlers and contributed to a lack of social reform that was experienced by all in the new republic. The belief at the time was that if laws and politically codified expectations could be legislated and mandated, then social reform would take care of itself. For example, if the Constitution could speak to issues such as Shays' Rebellion, then social reform would have been achieved. It only happens later on in the development of the new nation when individuals begin to understand that there are narratives outside the realm of one's own experience that have to be heard and acknowledged, and this becomes one of the major elements that initiates and enhances social reform.
Well, social reform is a fairly broad topic, so let me address the ones I would think most pressing at the time.
In the case of both women and slaves, they both participated on different levels in the Revolutionary effort. Slaves often did so with the promise of freedom in exchange for military service. Sometimes this was granted and sometimes not, but either way, the reform of individual and institutional slavery ran up against the racism of society (almost universally so) and the growth of slavery in the southern colonies. That is, there was a chance once the Revolution was complete to reform the institution, but economic (the exanded reliance on southern slavery) and social factors (widespread racism and fear of a free black population) prevented this.
Right after the Revolution, women felt there was a chance to build a new society where women, if not equals, at least had rights and a place to start down that path. Some women's schools were started in the 1780s, and Abigail Adams, wife of Constitutional Convention delegate John, famously urged him to "Remember the Ladies" while drafting the document. The powerful social belief, however, that women belonged in the home (The Cult of Domesticity) raising children to be "virtuous members of the new Republic" (Republican Motherhood) meant that by early 1800, women were largely in no better a social position than they were before.
I would say that the circumstance that did the most to promote social reform was the fact that the United States was a completely new country that was very different from other countries. It was the only country that was a democracy and it was the most multi-national country at the time. Both of these allowed for much more openness to change than would be possible in countries that were more set in their ways.
At the same time, though, there were clearly set hierarchies that could not be changed. Most clearly, there was little willingness to consider giving rights to women or to non-whites.