Well, I am going to change these questions a bit to work for me.
Whats the best SL thing you ever did?
Joining the Chknw group. This is the one thing that I did where I got to meet people...really meet people and hang out with them. Also, it introduced me to blogging and I feel like I know my SL friends a little better. Whether it be their SL or RL. I don't have to hang out in SL alone anymore.
What is the worst SL thing you ever did?
I would have to agree with Tymmerie and say when I made folders with outfits in them. That just made my inventory huge!! I had to spend a lot of time going back through them and sorting out the clothes, shoes, etc so that my inventory numbers would drop. What a nightmare!
What didn't anyone tell you that you had to learn on your own?
Ok, this is easy but finding the fashion blogs and the freebie blogs. I learned all about living in SL on a dime by joining groups and doing hunts, etc. I don't do those things as much anymore but I was obsessed at one point. I would wake up at 4am around Christmas to get all the prezzies before I left to work...Yikes!