So I guess I'm not ending my inquiry. Although of course we will be playing by Wesleyan (prison) rules I guess, Scripture, Tradition, Reason, and Experience, are all fair play.
The main issue of contention that has remained since the Reformation between Protestants and Catholics and is chronologically the first battleground, is nature (and thus grace).
What is Nature? Was it perfect without grace? Was Adam merely natural, or did he also have sanctifying grace? What is original sin? etc.
I'm going to try to keep posts shorter so that you don't have to read as much, with the sacrifice being extra prooftexts.
Pelagius argued that Nature was perfect in itself, and that Adam was not given any gift of grace, because nature itself is a gracious gift. Protestantism agrees with Pelagius in arguing that Nature was perfect and was without grace, it was "good", and man likewise "very good" ('no one is good but God alone' as Jesus said).
The Cappadocians and Augustine argued on the contrary that man was not without grace, and that the 'breath of life' implied more than life, but actually the indwelling of the Holy Spirit and/or Sanctifying Grace. God blessed them in the garden before the Fall, and this seems to me, to indicate a divine gift (grace). This is "Original Righteousness" in Thomas. It thus follows that Original Sin is merely "not having Original Righteousness/Sanctifying Grace".
Fr. Hardon (man I feel gross typing that name) has an article on it here:
Called to Communion also has good posts on it as well (here's just part 7 in the series: http://www.calledtocommunion.com/2010/03/aquinas-and-trent-part-7/)