This is a reiteration of other peoples' and some of mine (even though I don't play, I judge the shit out of other DM's because I've got way too high an opinion of my skillset):

1. "Wait... so what's everyone playing?" - *Sin: Neglect** - The DM that starts a game with little thought at all and just let's the players build stuff. No rhyme, no reason, no story to give them context, no interaction with helping suggest what things they take and might not take that will play more or less with the upcoming story. They end up an hour in and the PC's are poorly matches and related to each other, have no reason to work together, and aren't well made for the adventure ahead and the DM just shrugs because why should they help build a part with the PC's? That's THEIR job, afterall?
2. "Inspira-whaaat?" - Sin: Sloth - The DM that doesn't use the Inspiration system. Or at best they toss one out just when someone points it out or crits big instead of really leaning on the Backgrounds. Sure... the players could monitor and be attentive of their Bonds and Flaws, could play them and really mark the time with their character when they change them, and all of that rewarded by the Inspiration. The DM could be paying enough attention to know that PC-A has a flaw of cowardice and when he keeps hiding behind the rock and trying to use Help actions or bare support rather than charge the beast, and award him Inspiration for a great way to show it--but, who can be bothered? Who has the time? PC's are basically just video game characters someone else is playing... too much work to keep track of.
3. Oh, your third death save automatically succeeds because...." - Sin - Self-Esteem - The DM that saves PC's because they would feel bad for it dying and the player having to reroll. Most often, the DM doesn't feel his story is strong enough to compel interesting death. That death means his story isn't as good. That there can't be joy in the new and once he has a party working toward dungeoneering, any disrupt to that may derail the game. Confidence in a beautiful story and heroic deaths elude him.
4. ...rolls behind screen... - Sin - Fraud - The DM wants control over outcomes instead of circumstances. They could put in the work to have not only an encounter that is dangerous but also one that would make story sense at any point to stop. The lazy DM makes a dinosaur attack encounter. The clever DM makes a story where the PC's know dinosaurs (through Nature checks, local stories) are driven off by the smell of the undead (or the sound of the Horn of Whatever that the local cannibal tribe uses or some other predator like a young dragon or whatever). When the dinosaurs are winning too hard and the DM wants to spare the party? They can have the combat change with some REASONS and further plot (what undead monstrosity must be around here? That horn is amile away, getting closer!!! The sound of wings and the roar of madness heading to the mountain!) but NOOOOO, lets roll behind the scenes and lie because we can't be bothered to be more creative. That trap? Sure, you get hurt. That lie? Sure it fails. That thing the player wanted to do that isn't interesting to the DM? It botches... the dice can say anything.

I've got more... but I'll have to come back.