which aspects of islam would have appealed to the poor, enslaved, and isolated - many the first Muslim converts?

Muslims did not have to pay taxes, other than giving zakat-alms (mentioned in the Five Pillars) while Non-Muslims did have to pay taxes. The four classes of the Muslim Empire stated that Muslim converts were second class. These advantages would have appealed to the poverty-suffering Islamic converts.

Islam teaches that all humans are equal. A white man has no claim of superiority to a black man. The criterion of Superiority is Piety. All human beings are the off-spring of ONE and the Same Almighty Allah (God). He has created them and loves them all. This equality appealed to the slaves and the poor to convert to Islam.