Honestly, Diana was a human being with all the ups and downs and positives and negatives that brings with it. She was no saint but neither was she the evil villain. Diana did some incredible work that significantly changed the conversation around AIDS, land mines, etc. and she rightly deserves the praise that drew. However, we all know that Diana was certainly not all sunshine and rainbows. She was manipulative, volatile, immature, and all in all I would think she was incredibly difficult to be around much of the time. I do think that Diana's childhood certainly impacted who and what she became as an adult both for good and bad. That said, while Diana was in many respects an incredible mother, she certainly had her faults in that area, too.
As for why she continues to evoke such strong feelings, well, I think the only answer to that is that the spin-doctors were very, very skilled and did their jobs well. Not just those employed by the RF but those employed by the media, by the publishers of the book, by those charities and causes that benefitted from their association with her, etc. The public saw, read, and heard it all and the messaging both for and against Diana was extremely effective.
We can speculate endlessly about whether or not she'd still be alive if she'd remained married, toed the line, etc. or even if she'd divorced but remained quiet, discreet, and loyal to the RF. No one can ever know the answers to those questions with any certainty. I do truly believe that had she lived, she and Charles would have become friendly and developed a much healthier relationship. It is said that in the months before her death they were beginning to find a bit of friendship and were in a much better place operating as co-parents than they had been as unhappy spouses. We've seen Camilla and Andrew, Andrew and Fergie, and even Anne and Mark become friendly ex-spouses and excellent co-parents. I do believe that, with time and happier second marriages/long-term relationships, Diana and Charles would have fallen into this group as well. No one can know what their boys would have become and how their personalities, actions, and behaviors would differ from the current reality had their mother lived, though it is interesting to speculate. I do think, though, that we can safely conclude that that reality would be much, much different that the reality of today.