Has your life improved since you quit?
Yes, I now have time to devote to things that are more fulfilling. However, the beginnings of my dissatisfaction with tf2 coincided with the end of high school- since I was using tf2 as an escape, and once high school ended I no longer required said escape- so I wouldn't attribute the improvement to my quitting tf2. The extra time is quite nice, though.
Do you get bored enough to feel the urge to come back to TF2 often, or are you satisfied enough with where you are (or dissatisfied enough with TF2 when you left) to not even consider it?
I consider playing every once in a while, but I wouldn't ever be on a scrim-heavy team again. I would probably play casually with friends, but certainly not seriously.
Did you have some other, non-videogame hobby to spend your time on, or did you exit into the world with no clear plans for what to do next?
Preparing for college, preparing to move out, and pursuing interests like science and philosophy keep me occupied.
What was the main reason that you decided to quit TF2?
It was no longer fun, nor was it gratifying. The amount of effort necessary to improve to a point where I would be satisfied with my performance wasn't worth the unpleasant grind such an improvement would require.
And, of course, why do you browse a forum for a game you no longer play?
I still enjoy watching the game and reading the drama.