GMing can seem like a thankless job, but people do it because it's fun. Fortunately really bad GMs will winnow themselves out, driving away players. But good GMs can still engage in bad behaviors, I know that I've done it.
1)You are not the players' adversary. It is not your job to screw over or wipe out the players.
2) Don't play favorites. This goes double if you play with an SO or spouse and triple if you have a romantic interest in a player but no relationship.
3) Don't pick on players/characters. Yes, someone at your table may be irritating or you're annoyed at that rules lawyer. That doesn't mean he's suddenly wearing Axe for Monsters.
4) PCs should be the stars. The players aren't there to entertain or provide an audience for your NPCs.
5) Don't hinge the entire night on the players figuring out something that you think is obvious.
6) Don't run if you aren't up to it. GMs usually feel a certain sense of duty that makes them feel obliged to run a game even though they physically or mentally aren't up to it. Players would rather take a break, have a movie night, or play something else than sit through a sucky game.
7) Don't railroad. Even if you spent hours crafting the Dungeon of Woe, if your players decide they'd rather visit the village of Hamhock, let them. The dungeon will still be there.
8) Don't keep your players in a hopeless situation. Drama is good, boring despair is not. Role-playing through the characters' prolonged imprisonment with nightly torture is no fun. Time for a cut scene.
9) Be consistent and fair.
I know I haven't hit them all, but these were at the top of mind.