I really think that virtually nothing good comes out of Slytherin. Almost the best you can hope for is evil but lazy and self-interested (a la the better Malfoys). In Snape's case, remember what Dumbledore said about sorting too soon?
Harry is almost too good for his own good ... I believe that is his true nature. He does have a bit of that "I'm on a mission" streak where he doesn't necessarily respond in a fully human way, but I think that's a natural result of being "anointed" for a particular purpose.
I have not reread anything yet, though, so perhaps that could make a difference. I thought JKR's Q&A made her view of Harry pretty clear ...
I want to read the last book for sure again because I thought the plotting around Dumbledore's death was so weak. Unfortunately a friend got me started, so I don't have all the books ...