I'm going to answer this as "What have you learned about yourself from the forum" instead of "What have you learned about yourself through MBTI", because I have a hard time separating my MBTI-related personal growth from my overall growth as a person since I joined, because I discovered it when I was a rising high school senior and now I'm a college junior. A lot has happened in my life, you know? And I don't know how different a person I would have been if I never knew about the MBTI.
I think, since I joined this forum, I've discovered that I'm wiser than I thought. I didn't know before because I wasn't very good at articulating my deepest thoughts and values, but I've gotten very good at that since I joined -- and I think that easy expression of my Fi is why some people on the forum suspect that I could be ENFP. The way that I post -- i.e. stream-of-consciousness with a minimum amount of proofreading and then posting immediately -- is very similar to how I might talk in real life (and it's why I'm not all that eloquent on the forum!), which means that any skill I've gained at self-expression, through this forum, has carried over into my everyday life. Also, the degree of "thinking out loud" that I've done in this process -- especially on the "Ask an ESTJ" thread -- has really allowed me to figure out what my neuroses really are, how I operate, etc. And all the debates on this forum have helped me realize that I always need to pick my battles, and sometimes I just have to let go and let crazy people continue to be crazy. (I'm still learning this lesson btw...)
I've learned, through my friendships on the forum, that I'm nicer than I think I am. I thought initially that it was just because they "don't know me well enough" to see me in douchebag mode, but then I realized that all the conflict resolution/moderation/avoidance that I do on the forum, I often do in real life as well. (The difference is that I open up to people quicker here because there's no awkwardness.) Because I was such a domineering, obnoxious, and arrogant person up until maybe junior year of high school, I've been actively trying to be a nicer, more level-headed, less hot-tempered, more reasonable, and less irritating person, and the forum has shown me that my efforts are succeeding so far. Obviously it's an ongoing process -- isn't all personal development ongoing? We're all striving to be better -- but it's good to know that I'm, at the very least, on the right track.