what is winstons only redemptive quality at this point? what does this hint at?

Your pages don't match mine but I think know what you mean. O'Brien has always been interested in Winston because of his intellect. If O'Brien was not so interested in Winston, he would have vaporized Winston long ago. I think Winston's redemptive quality is his keen ability to reason. O'Brien is hoping that with the right re-education (torture) Winston can be "cured" of his illusions. Give me the chapter you are on and I'll double check.

I am on Part III, chapter 3. Thanks

Yeah, I think it is Winston's intellect, his power to learn. O'Brien wants the challenge of "re educating" Winston which is a terrifying prospect.