I've played with some very bad DMs.
Pet NPCs (including a sentient, invincible giant rat).
Obvious railroading.
Deliberate misapplication of the rules (the latest one was in star wars d20, where some droids apparently can do AoOs with ranged weapons, shoot with both their weapons as a partial action, or shoot you while grappling instead of making a grapple check... unless they are PC droids, of course).
Blind application of patently stupid rules.
Inventing new equipment of powers that are about ten times as powerful as what the manual describes as "the most powerful weapon ever devised".
Handing out excessive amounts of treasure.
Handing out excessive amounts of XP.
Introducing characters and elements from the latest cool manga/movie/whatever regardless of what we are actually playing.
Needless fudging of rolls and NPC stats (ie, cheating).
Killing the campaign every month or so.
Telling everyone to make complex and well-detailed characters because we're going for a long-running, deep-roleplaying game, and
then killing the campaign after a month or so.
And still I've never walked away from the game or the campaign. I'm a pathetic RPG addict.
I've been guilty of several of those during my DMing career, and it took me years to get rid of all of them. The trick is to admitting to it in the first place. Once the truth is firmly in place in the DM's head, everything will improve.