[tk75x]

Knowledge of good and evil is a "Good Thing", and one interpretation of the reason God put the tree in reach of Adam and Eve is that he wanted them to eventually partake of its fruit, but they did it early and without whatever other prep was necessary. Think of it as a child slowly learning concepts instead of being immediately presented with complex knowledge and not having the necessary foundation to process it. Not sure about the positive light depiction, but I could see hallucinogenics being viewed as something that expands your knowledge/mind especially since people report out of body experiences when using them.

[mandeville]

Ditto. Also in traditional doctrine it was good because the fall led to jesus. Look up felix culpa on wikipedia. As for the mushrooms, it's not as if medieval clergy didn't have a good time.

[iammisc]

You can't mention Felix Culpa though without mentioning Original Sin. Like most things in traditional Catholicism, there is a duality and a paradox. You can't get one without the other. Mentioning the Fall as a strictly good thing is simply ridiculous. That does not find any ground in traditional Catholicism, which also believed (and still believes frankly) that all the bad things in life, including the pain of childbirth, disease, sin, and destruction, are directly attributable to the sin of Adam and Eve.

Plus, there are several translations of it. The common rendition into English is 'happy fault', which may be too positive. The other translations include 'lucky mistake', 'happy mistake', blessed fall, etc. Note again the duality, the paradox, and that the fundamental noun (fault, mistake, fall) is negative. It's a paradox, a mystery, meant to get you to think deeper, not some stunning endorsement of listening to Satan.