Things that you've considered "bad DMing" from other DMs that you make sure to never make the same mistake in your game?
For me, it's introducing powerful NPCs way too oftenly (in my current campaign, our DM introduced a bunch of them, and sometime it's like watching the DM playing with his toy) and make too many combat where you "intended" for the players to run from it (1 or 2 time maybe fine, but from the 3rd time foward, it started to get annoying)