The most I have learned from foruming (on some other forums too) that how stupid I am (in communication, understanding life things, reasoning and arguing, etc). You can judge by my bullshit posts. The most important thing that I SHOULD have learnt by now is that I don't have to write those BS no matter the urge to "express myself".
Game developing-wise, well, I'm not sure. I had already been doing that for about 5 years when I joined, and I have just been less and less active since then. And when I joined, I didn't have too much questions as I was always obsessed with solving stuff, reinventing the wheel myself. Thus it would be hard to trace my growing only from my posts.
The most spectacular development was my written English skills. It still sucks grammar-wise, but I can write technical texts pretty effectively.