For instance, monsters in a D&D world are often an omnium gatherium lacking any rhyme or reason. Why do you have ghosts and mummies and ghouls and vampires? Because they are in the rule book and can be used, and what the players want is adventure, not rhyme or reason. It makes the world weaker.
Not to mention that the D&D approach toward creatures is distinctly combat oriented. The famous joke about the characters who go to kill the gazebo is classic for a reason. You get such things as the attack of the banshee, because the original banshee is hard to make significant in the game; lamenting the dead, and because she has foresight, lamenting them the day before they die, can be dramatic in a novel, but doesn't work so well in a game for many players -- if only because they don't have a role to act upon.
I mentioned last time that you need to focus on the player characters evenly and not let a non-player character steal the show, but this time I want to recommend Order of the Stick and Rusty & Co. as examples thereof. Both webcomics are actually set in worlds where the characters know they are in role playing games, but both have a focal character in the party that gets more attention than the rest -- and non-player characters who not only do really cool things but often do them when the player characters are off-stage.
I did not mention last time the beginning issue of teams: how to get the characters together. Quag Keep brings this into focus because what Andre Norton did was import a group of players into the D&D world. Rusty and Co and Order of the Stick have parody origins that go basically like a kludged together excuse (You All Meet In An Inn!). Convincing groups need more. A random group of people, caught in the inn, would have to forced to go on an adventure together -- magic or trickery to bind them together -- because they need some motive.