Hardest lesson to learn as a DM / GM?

[adembroski]

• • Know your rules inside and out.
  This goes without saying. If you're going to run a code breaking game, you don't want to practice sticking to it.

• 
  
  It seems like this would go without saying, but honestly, there is no substitute for being an expert in the game system. The confidence that comes with feeling like you know it inside out allows you to be a better DM in more ways than just being able to make spot rulings. It's just invaluable.

 

[FarsightDesignChris]

I think the second of the OPs hardest lessons is the one I relate to most. I remember always trying to put in a bunch of puzzles and complicated back stories when I first started running games. Those things always got derailed or were just hated. I had to find a balance between exciting combat adventures and thinking. Basically, the hardest lesson was realizing that the DM can't run the story just the way they want. I think it was a good life lesson for a kid.

 

[jeffh]

The hardest thing to get right is pacing. I'm still only okay at it, I think.