It's interesting that a kind upbringing can have such a large effect, I'm not sure this makes sense-- why wouldn't more of Quirrell's misanthropy have carried over?

Eh. I never got the sense that Harry was that good in near mode. Consider him not seeing a reason for Ron to exist, his total lack of interest in Hagrid, the incident in Diagon Alley, "birth parents," and the list goes on and on--but the difference is that while Quirrell is perfectly willing to jump from "I don't see a reason for Ron to exist" to "I can kill Ron if I feel like it," Harry has been taught that's not how the Right People think about things. But does he value Ron, or see a reason for him to exist or be happy? Not really, nowhere near like Hermione does.