Nellisir said:
Most common? Being too complicated, too complex, and holding too much back. The players never ever have all the information that you have. Complicated DM schemes never work. Also, don't hold stuff back. It's the surest way to make sure the campaign ends before you get there. If you have a cool idea, go with it. Make the game FUN for you AND the players. Make sure something happens every session.
A corollary to this is an error I see a lot of DMs make (and which I have made in the past, but try to avoid now) - designing backstory and campaign material that never affects the players. This is not, in and of itself, a problem, but it is often accompanied by a lack of development in areas that
do affect the players which
is a problem. I think the problem stems from published adventures, where writers seem to relish in giving this sort of unnecessary detail.
"The farmer dislikes his neighbors" - that's nice, but who cares, the players aren't his neighbors, and unless there is something more, they won't care about this sort of village dispute.
"The orcs have recently elected a new chief" - great, why do the players care?
"The evil cult is framing a group of bandits, who don't appear in this adventure, and have all been killed and their bodies sunk to the bottom of the swamp, but here are their stats and personalities" - if they don't appear, why bother detailing them?
"A thousand years ago, this altar was used for sacrifices to the rat god, in rituals that . . . [insert three paragraph description of the rat god cult] . . . but no one knows anything about this cult any more, and the only thing remaining is this altar, which cannot be identified by anyone."
And so on.
And then you get incomplete material that
could be useful:
"The curate is suspicious of the traveling priest" - Really? Why? If the players ask him about the priest, what will he say? Does he have specific things that make him suspicious? Did he see him do something odd, or hear him say something strange?
"The barkeep has heard rumors about the abandoned keep" Okay. What rumors? That is exists? That it houses an evil tribe of goblins? That is has pink bunnies living in it? Give something concrete.
What a DM needs to do is make sure that they prepare the information that will be of use to the players - "the shopkeep is suspicious of the traders next door" is not usually useful. "The shopkeep thinks the traders are up to something because he has seen them having late night meetings by the stables with people he doesn't recognize" is a better background.