I feel that the single most important thing a Mormon should learn is that the experience that he or she interprets as the Spirit confirming truth is not a reliable way to identify what is true. Or, just because you have felt what you call the Spirit it does not mean that there is no possibility of being wrong.
Now, why would I say this is the most important thing Mormons should learn. It has been my experience that Mormons will not consider policies or doctrines on their own merits if they believe the Spirit has spoken. The only thinking that can be done once the Spirit has spoken is in demonstrating how the policy or doctrine is true. Once one is uncertain about one's knowledge then one can begin to hear and consider evidence that runs counter to the Church's teachings and positions.
Mormons need to quit treating women as inferiors, promoting homophobia, arrogantly looking down on non-Mormons, stop trusting their leaders so much, etc. All of these things will not begin until they consider that current policies and doctrines might be wrong, and that will only happen once they realize the Spirit is not a reliable way to identify truth.
What are your thoughts?